All Done, he used the part a bought which was £60, labour was £84, happy with that.
He says I was going about it the right way, because usually the cartridges pull out and push in quite easily. But he's never seen one so corroded enough, to be jammed in.
His solution was to take the whole valve off the pipe, open the connector at the opposite end of the cartridge connector, then, hammer it out. Was just a matter of a bit of brute force.
Most of the time was taken up with draining and filling the cylinder.
You all were correct, i really was way out of my depth here, i was daft even thinking I could fix it.
Better still, he was able to tell me all about the system to get the best out of it, which will save me money on electricity.
Thanks for the advice lads, I took it in the end!
Cheers Tony.