Thanks for that Jocelyn!
I will work through your suggestions over the next while. For the moment I have started experimenting with holding the fiddle in different positions and I’m surprised to notice how even minor changes can affect both the muscle strain and the playing itself positively or negatively. I also got a different shoulder rest (Wolf Forte Secondo) which is quite adaptable and feels a lot more comfortable. Finally I have changed the position of my chinrest (didn’t know you could do that!), moving it from the side towards the middle, which has definitely helped.
Well, as I say, I’m experimenting and it’ll probably be another while until I find the perfect combination (and that in itself is probably something that keeps evolving) but I’m definitely making progress.
Thanks again.