Thanks mods for the move, this is where I wanted to put it.... but thought I was wrong.......
As an update: I found the part on eBay and messaged the seller for information on how to remove the old seal. He said to replace the seal you must first remove the shower screen. If you cannot achieve this then you cannot replace the seal!
This is first problem as I have tried to remove the shower screen before and failed. The screw is still in good condition, with a bit of wear to the head but still heaps of purchase, I just cannot for the life of me get enough leverage on the screwdriver.
Any hints, tips, ideas etc welcome!!