My preload adjuster just seem to spin endlessly in either direction, is it buggered or is supposed to do this?

Sounds buggered. You can take it off for a look without anything exciting happening.

Do you need to adjust it at the moment? If not I wouldn’t worry about it anyway

It’s not perfect at the moment but it’s not far off, not sure I’m brave enough to start taking forks apart, might just leave it as is!

You wouldn’t be taking it apart, just taking the top cap off. The most dramatic thing that could happen would be the fork sinking so that the spring stuck out the top of the leg. If that happens, just extend them again to get the cap back on

Just taken the cap off, I take it when I turn the adjuster the bit underneath should extend or retract? It just spins?

It’s buggered, one of These is what you need..

Taking the topcap off easy with a 26mm socket and is a good opportunity to check and top up the fluid. I’ve learned the hard (& expensive) way that dry seals lead to stanchion wear.