Obtaining a bespoke size pocket sprung mattress

We have just purchased a brand new static caravan which has come with a basic sprung mattress. Also the bed size is 5' x 6'3", which is not standard and cannot be changed. So we are looking to purchase a comfortable 1,000 pocket sprung mattress with these dimensions. It seems an absolute minefield!!
Any help you can offer would be so appreciated.
Our budget is a maximum of £350
Many thanks,
Mr Kim Lewis