Hello titfer! My sumo babies rarely fit into gap stuff, American apparel or petit bateau. Also la redoute measures the continental way by cm and even with that I usually have to double what I think will fit so they might work better for you.
I still struggle to get ds2 trousers as he's really slim at 21 months and now have to but adjustable waist ones from next, tops are fine but trousers I get from there. This year all the trousers seem to have got even bigger in the waist! When he was little (he was 4lb 14 at birth) I got babygrows from mothercare.