Marks and Spencer's kids clothing is usually pretty generous in size. I can fit into their boys size 15-16 pyjamas comfortably as a stocky 5'8 man. I can get into their size 13-14 boys pyjamas at a squeeze too.
The special needs clothing is generally a bit more form fitting to conceal things like nappies but I can fit into their size 15-16 onesie although I haven't tried their sleepers.
I'm really glad that Seenin are now doing size 15-16 in their sleepers. I think until recently they only did size 13-14 which looked to be too small judging by their size guide. I cannot wait till they are back in stock as I will definitely be ordering some.