When the window reaches the top of it's travel, I get a clunking sound like a cog is jumping a ratchet and I have to let go the switch because it sounds like something will break. I suspect I need a new motor, but can anyone confirm?

Because of this I can't reset the top and bottom points, so every time the door is opened the window drops further down and I have to remember to carefully wind it up again before I get out. Luckily it's the passenger window.

Thanks in advance.

 

Did you check the FAQ's for Reverend Sam's videos? He has one troubleshooting a fault which sounds a lot like the one you describe:

Jaguar XK8 window adjustment. - YouTube

If you need advice on removing the door panel, he has another video covering this:

Jaguar XK8 Door Panel Removal. - YouTube

You don't say whether you have a Coupe or a Convertable but the window limit principle is the same for both.

Graham