Following advice kindly given by Topcat, I fitted a pee zip from Lomo to one of my Decathlon "Olaian" suits (for which I'm still grateful to pwhole for pointing me at) and tested it yesterday. It works fine and does not leak water into the suit. However, it is vulnerable to damage so I also wore the top part of a cut off 2 mm thick Long John over the wetsuit (think vest with a crutch). This protects the zip and only the Long John needs to come off when the pee zip is used, not the whole suit. The Long John makes no difference to ease of movement in the (stretchy / comfortable) Decathlon suit.
To fit the pee zip I used the proper 2 part glue (also bought from Lomo) and reinforced this with a thin bead of Aquasure all around the edge of the zip tape on the inside of the suit.
This is the best wetsuit combination I've ever used for wet caves and I'm very happy with it. The addition of a thin hood attached jacket over the top allows reasonable length dives in comfort, without the mather of caving to the sump in a proper (restrictive) diving wetsuit.
There is a back zip version of the Decathlon "Olaian" wetsuit, which is considerably cheaper than the chest zip version I have. I'll wager it's also not quite as warm as it lacks the quality neckseal arrangement of the chest zip model. Here's the link to the cheaper option:
This wetsuit is easy to put on, so you can get dressed quickly and comfortably during your winter sessions.
www.decathlon.co.uk
My own preference though is for the more costly chest zip version. Remember, if the back zip version is chosen, you may need to order a size bigger than suggested by the sizing chart. (I'm a 6' 2" giraffe and, like pwhole, I buy "XL".)