Quick update, think I've sorted it.

Reckon it probably was an airlock, I got the car up to temp, turned it off then carefully removed the cap from the expansion tank, then restarted.

Ran it without the cap for a few minutes, after a while I could feel both the radiator hoses getting warm, indicating that the water was flowing.

Replaced the cap and went for a spin, both hoses stayed warm, hot water wasn't bubbling back into the expansion tank and the cooling fans didn't come on after just a couple of miles.

So, fingers crossed, looks like its sorted then.