Thanks for all the thoughtful responses guys. Let me take it point by point..
Claudiu:
I had the same thing since before stream entry. Personally I think you are (and I was) semi-consciously causing the muscles in your head to contract... in other words, it's a way to cause yourself a Tension Headache. I would not recommend making having a tension headache the norm of your day-to-day experience. It was mine for more than a year and it sucked!
That being said, if what you want is more concentration (as in the 2nd training), then a persistent, on-going tension headache might indeed be very helpful. It's a very obvious sensation that you can notice whenever (and you can get to the point where you induce it whenever you try to pay attention in a meditative way), and it is a physical thing that hurts so it provides a lot of distaste for the world, along with a desire to end pain by withdrawing from said world. Both of those lead to concentration, as concentration seems like a way 'out' of the pain/unpleasantness... never mind that you are the one inducing the tension in the first place.
In daily life this tension is not really an issue. Sometimes trying to sleep it will come on strong but I just touch my forehead lightly and it goes. When I first read your reply I thought we were talking about 2 different things. Or perhaps the same thing, but with an added bit in my case. After a few sits it seems that I cannot find this "solidity" inside the skull. It is indeed just muscle tension. This is surprising to me, but good stuff. Thanks!
I should also point out that in EQ it is not often unpleasant. Just
there. In regard to sensations to concentrate on, I'm with Nick (and you! heh..), this is not a good option. I have such strong vibrations in the body whenever Im still anyway. I usually watch those, or if Im moving, the breath and/or grosser sensations in the body.
Simon:
You might be interested in this thread:
http://dharmaoverground.org/web/guest/discussion/-/message_boards/message/3218934
Got it open, will read. Thanks!
Nikolai:
I would realise how much one's focus is jumping on such sensations giving them mental weight, shape and the unpleasant tag.
Though I think it's a bit of an academic point, I must mention for the sake of clarity that the sensations across the forehead is not unpleasant. It can get that way for sure, but most of the time it is just "sensation" whilst in EQ.
Nikolai:
The actual anapanasati instructions call for a fullbodied approach to concentration. Try seeing if ignoring the sensations in the head works while shifting focus elsewhere (this will probably show how the mind's habit of lunging on an aspect of experience and giving it special status and shape is firmly in place) and/or widening perception to juxtapose those sensations in the head with the rest of the whole body, and/or seeing how the periphery of vision is a softer relaxd focus compared to the centre of vision amd allowing the periphery to inform how the centre is perceived visually thus taking in the periphery and centre at the same time.
This is really interesting stuff Nick. My practice is pretty much full bodied awareness - using the whole body as wide focus point. This means "whole body full of pleasure", "wholebody full of sensations", "sensation soup"... and the head tension is always present to a lesser or greater degree. I do not see my mind darting to the tension, or averting from it. (though this could well be happening of course) It's just part of the whole thing, though apart in that it vibrates at a different rate and has a sense of solidity where the rest of the body feels like a vague gaseous mass of sensations.
I've tried focusing on the periphery as you said. I need a few more sits but initially it seems very promising indeed. My natural centre point is from behind the eyes so when I try this it either does not affect the head tension, or even increases it (though it could of course be just increasing on it's own). However, "looking" straight but paying attention to the edge brought some interesting shifts in perspective that I need to explore more thoroughly.
The really promising thing was when i moved my attention to the chest, but kept full body awareness (it seems impossible to drop that anyway) AND paid attention to the periphery again. The tension lessened quite a bit and I had similarly interesting shifts in perspective as I sat. Way to early to say this works, but I will work with it over the next few days when I reach EQ and see where it leads!
Nikolai:
When this is mastered, it can then be applied to any and all sense doors. Experiment with 'peripheralising' those centre sensations in the head, includng the perphery of the physically sensed universe i.e. the entire field of the body i.e. full bodied. Experiment with getting 'concentrated' or rather calm and collected soone can then see how craving and aversion works (hint: they need the mind focused on and thus creating a mental object with shape and weight, such as the 'shitty sensations' in the head.)Experiment with ways of ceasing such a fabrication by playing around with how such fabrications are given mental weight, shape and label.
Right now i find it hard to even imagine peripheralising hearing for example, but just doing it with the body seemed hard the first sit, but the second one got easier and there's a kind of knack to it that I am starting to grasp. So I will read this paragraph again a few times and try to really understand it

Nikolai:
Doing such a practice may well help synch things up in the 11th nana as any 'object' the mind gets fixated on even subtly, segregates and ignores the rest of the field of experience, and in the 11th nana, it becomes more inclusive or at least leads one to take more in i.e. sensations of here & there, me & other, observer & observed, at once , mashed and mixed up in one borderless mass of phenomena. The peripheralisation of the entrie field of experience.....and then relax and let that go too.
Yessir! On it.
Dauphin:
This may not apply to everyone, but in my case what brought about SE was that I finally gave up trying to see or find or identify anything in particular. I was relaxed, but perfectly alert, totally content observing sensations and my "awareness" of those sensations. It was just how/what the world was. Then, when I least expected it, I found out about the true nature of that "awareness."
God i hope that works for me too! Seems that when i start trying to figure things out it just gets me unconcentrated and feels all wrong. When it's good in EQ i just keep following the vibrations. Every time they speed up a notch Im left wondering just how deep can this can go? Sometimes it seems it will never end, and yet the body/mind feels it cannot go any further. Then after a while theres a shift and things speed up / change once more and the boundaries change again! Intuitively I have been trying to just let go into that, trying not to manipulate it, or cling to it. Does that make sense to you Dauphin?