Leaking Aqualisa shower - can I just replace the seal?

I've got a leaking Aqualisa shower. I have removed the cartridge - this was not an easy job because the two lower bolts had rusted away and I had to drill them out. I think I managed to save the cartridge. Now I have to decide whether or not to buy a new cartridge (£65 - £110 depending where you buy it) or just replace the seal. I'm in favour of replacing the seal (£3.00 approx) as the shower temp was ok even when it was dripping but I do need two new screws to replace the ones I blasted out. So - question, do I just buy a new seal? and where can I get some new screws? Thanks in advance for your answer.