Dear Dextah

From ur description it seems to me that you have built trigger points (TP) in ur trapezius.
As u mentioned that all your investigations are clear. Also taking ur aggravating and easing factors into account; Using ur arms brings u pain. The reason,when doing movements muscles contract that triggers the trigger points.Also you have got tender spots along sides of spine causes pain when pressed. TP are common in trapezius in people who use to sit hours in front of computer, which u used to do. Also u mentioned loosing weight by exercise and diet. Overdoing exercises can be responsible. If other muscles are weak, the trapezius will often try to ''help out'' even when it strains itself. Studies showed that poor nutrition also aggravates TP.
Having TP is an irritable and painful experience as I have experienced same in my own body.But I request u not to loose ur hope in physios.
TP can be treated by releasing them through massage(specific) followed by heat therapy. Some physios expert in manual therapy can do it with very positive results.Some physios use therapeutic ultrasound also. It can take a little long to get better and also TP release can be a painful experience.
As TP sometimes cause movements painful and thus restricted. So I suggest along with TP release massage, Mulligans NAGS and reverse NAGS for cervical and upper thoracic spine respectively, which gives wonderful results.
I suggest u some measures which could help:
Good nutrition like vitamins and minerals.
Use good body mechanics.Like when sitting at computer,make sure that ur chest is lifted and shoulders relaxed away from your ears.Keep ur head in line with the rest of ur spine.If ur head leans forward(as u mentioned) the trapezius muscle has to contract to counterbalance. When u use good sitting posture, the trapezius muscle can relax.
Smoking aggeravates
Being chilled can aggeravate, so stay warm.
Wear comfortable clothes. when the muscles of the top of the shoulder gets compressed by a heavy purse, or an ill fitted shirt or coat, they react by tightening.
Hope it helps...