I've had usual aches and pains (muscles aching I didn't know I had) but that fades quickly. This pain comes later, which is a little weird.
The main difference is that I've been on call lately so I've been lugging a laptop around, which I sling over my right shoulder, for two weeks on and off. This will no doubt have aggravated the situation if the rucksack has started it off. It's really the muscle between my neck and shoulder which makes me think it's load related (laptops being especially one sided).
I'm off call for three weeks so if the new rucksack doesn't give me the same problem it looks like a combination of new activity and the existing strain the laptop provides.
Next walk is fairly easy (Lathkilldale) so I'll see what happens. If not off to the docs. If it does go, work will have to supply something better to carry the lappy.
Thanks all.