Well I have finally got some that fit!
I'm in Scotland so less than a week to go - was beginning to panic. My DS has the bottom and waist of a 4 yrs old but the legs of a 6 year old. I have tried school trousers from most places - all to big and baggy - way too much material.
Found 2 good options at M&S - both ordered online:
Size down (to age 5) but buy longer length in these longer length M&S
and these "modern fit" aka skinny bought in regular size modern fit
I think the latter are the best fit.
Thought might be of use to save others the trecking around that I have done!