Any ideas of what may be going on to allow the rads
to keep getting hot, with everything switched off,
would be gratefully received.

Dodgy zone valve?

Alternatively, and I don't know the design of the boiler in question, the
radiator circuit may be pump controlled rather than valve controlled. If so,
there might be some parasitic gravity circulation, although you would only
expect the radiators to warm up when the hot water was actually being run
then.

If the design is actually a small heat bank or similar, this may also
gravity circulate and even cause boiler firing on its own accord.

It may be an idea to post the model of the boiler to determine how its
control systems are likely to work.

Christian.