Pulled up at my destination today with a pile of smoke coming from under the bonnet. left it to cool and then started up again. sounded like bearings on the air con compressor with the excess resistance causing to belt to smoke and throw bits of rubber off. The pitch increased with revs. When I turned the aircon off the noise stopped.

Any ideas on narrowing down the problem?

thanks in advance.

Could be your aircon compressor.
but also check the tensioner pulley on the aircon, as these seize. It should be easy to slacken when removing the belt.
mine did exactly the same, compressor I believe, gets strained if the pulley gets seized.
pita job as the aircon will have to be discharged correctly, and if pulley needs to be changed, it's right by the bottom of the rad.
Greg.

Thanks. It is definetly related to the aircon switch so I will have a go soon and investigate. Just in time for summer