Thanks for the speedy reply, not sure how I could have missed that tickbox!
Just tried it, and performance was back to perfectly acceptable, so it's the caching process that seems to be the problem. I'd still be interested to know why it was being so troublesome if anyone has any ideas I'd still be interested to hear any suggestions. This PC gets used mainly for audio work, so there is a lot of big files and data caching going on and I want to keep it working well.
While I'm here, does anyone know if there is a way to verify a copy that has just been burnt when you've forgotten to tick the box before you started! (Or a way to get the verify box to stay ticked by default?)