Splash about (sell on Amazon or own website) have various wet suit material for babies. I used the all in one wetsuit when I take my 3 month old swimming and it keeps him nice and warm for the duration!
Ask how warm the pool is. If is cooler - below 30 I would say but no expert - you might want more of a wetsuit (on top of swim nappy and happy nappy).
Also check with the pool what they want babies to wear - the place we go, they were very strict with this and will only allow babies in with the happy nappy
Thanks all for the advice! I have gone for a swimming costume with a nappy and a then a disposable nappy I think! Much appreciated!!
Huggies then the disposable nappies and then a happy nappy over the top (Amazon sell both)