I spent ages looking for a bed to fit an odd sized alcove in DS's room with no success so had to come up with another solution. His new mattress has just arrived and I thought it might be handy to post this for anyone else who has the same issue in future.
We used a normal sized wooden single bed. DH cut the end off and bought two new bolts to bolt the footboard back on. He had to make a mortice and tenon joint (think that's what they are called but managed so it can't be too hard as he's not great at DIY (sorry DH !).
New mattress has come from Babymattressonline.co.uk who had good reviews on other sites when I googled them and I found a 10% voucher. I ordered the bamboo one as it was the widest one in their made to measure range, it fits perfectly. He had a memory foam topper which I've cut to size and that's on as well and he should be really comfy and now has a lot more floor space now I've been able to rearrange his room. Mattress was ordered 18th Jan and arrived today so pretty quick in scheme of things. They say you can mark it as Urgent if you need it very quickly.
He could do with his duvet cutting down to fit which is next job but won't be end of the world if I don't do it. Total cost for us was about £108 which was £104 for mattress then about £4 on bolts for bed, would be less if you chose a different mattress.