Thanks for these answers guys. While I was waiting for the new bed to arrive, I messed around with the old one to see what i could do.
I was wary of removing the old heater, as I wasn’t sure if I would break the heater in doing so. As a result I decided to try and see if there was some way of keeping the heater mat in contact from below. The first problem though was to get rid of the bubble, which was full of air. In the end, using a new scalpel blade, I made a small incision in the mat between where the heater wires obviouly go, and then pressed the bubble until it was all back in contact.
I then used a bit of 2.5mm thick rubber foam sheet, and cut it to size and lay it under the bed. The rubber I used is able to take up to about 150deg C so it was safe. The result is that while you can see that the heater is not in 100 percent contact, as there are small areas that are still showing bubbles, it does keep enough contact to work and keep a stable temperature.
Not perfect, but at least I can use it as a back up.
Of course, now the PEI film has started to bubble too (maybe due to local overheating?) but as the new bed came complete with PEI film that’s no issue, especially if used as a backup