We have bought a super king size bed (I know some shops bed frames come up in slightly difference measurements) however the mattress, which we didn't buy, looks slightly small around the edges due to the style of the bed frame. Do you know anywhere that sells slightly different/bigger/made to order mattresses? We have looked at all the local bed stores and IKEA as I know their bed/bedding sizes come up slightly different, but have had no luck. The bed is 216 wide by 221 long so ideally would need one around 225 by 230? Do they even exist?
A superking is 6ft 6 long (200cm) by 6ft wide (180cm). Based on the dimensions you've described I think you've got a large emperor sized bed (7ft by 7ft). It is possible to buy mattresses in that size but it will be both incredibly heavy and difficult to manoeuvre - you might need two mattresses that zip together. I would start by googling for large emperor. It is possible to have mattresses made as we have had them made for a couple of non standard beds but can't remember the details - in both cases it was a local mill type place; one in West and one in North Yorkshire.