Has anybody got a little oversuit for sale, to fit a child 110cm tall, who normally wears age 5-6 clothes? Not one of those they sell in Bernies though - they're a really weird shape.

Decathlon sometimes have childrens shortie wetsuits. They are not always in stock (like most things there!) so it's worth a ring before you visit a store. The last time I saw some in Nottingham, there was the size you suggested. At the price they were, I bought one for my daughter at her current size, and one at the next size - £12 each! She's very thin and so wears a furry and oversuit over the top to keep warm (both from Daleswear)
Good luck with the search.

cheers, thanks Dave. We've also found that keeping warm is a problem - a case of all surface area and no volume!