For anyone else looking for them, I had some great help on here so just wanted to share where I eventually found them to help any mums who have arthritis or hand problems that make poppers a nightmare.
Right now they have some lovely packs of pjs in Tesco, some snuggle sleepsuits with zippers in Asda, there are some zipped sleepsuits in H&M in store not online unfortunately and not many and sadly you have to be lucky with sizing as they don't seem to hold stock and there is one zipped sleepsuit in Next for newborns. Mothercare also stock newborn pyjamas but they are very expensive compared to the supermarkets.
Hope this may have helped as I have been looking for months and finally they seem to be coming into stock. :)