Thanks for all the replies, they will all be useful when I do my write up. I've got 3 weeks of scope posts to write first so unfortunately the pellet post won't be any time soon.

I managed my first trip to Iden Ferns today (great club btw). I tested my air arms diabolo 4.51 vs 4.52 and as predicted, I couldn't tell the difference. However JSB express were a definite improvement in group size. This is the first time I've tried express and I'm impressed so far. Although it was a very still day so I don't know how wind will affect them yet. (This is with a TX200HC, almost standard internals with just a bit of weight added to the stock).