The problem has been solved!
A few days ago the problem deteriorated to the point where it would squeal when revved in neutral, so I looked in the engine bay with the engine running and confirmed that it was the one of the drive belts - most likely the water pump and alternator belt.
The belts were tightened and now there is no more squealing!
There could have been an as yet undiagnosed cause such as a problem with the alternator causing more drag, but for the moment the problem is solved.
Thanks for the tips btw.
Cheers
Hugo