I've just had to replace my alternator and belts, I ordered an alternator belt (ETC7939) but it's thicker than original, same thickness as the PAS belt, I've had to bung it on for now as the old one was disintegrated but it doesn't seat flush in the pulleys, does anyone know a part number for a slimmer one or will this one just be ok?

..... also, viscous fan on continuously, should I worry?

The oe spec 200 belt is quite thick, but its to thick for the pulleys and wears funny, I just ran a std 3/8 9.5mm v-belt, sorry dont know size.

The fan if by on you mean its spinning? if so thats normal, its how it spins hot and cold that makes it right.
Cold engine, engine switched off! if its stiff to turn by hand and stops almost straight away after letting it go its okay, engine running for a couple of minutes try and stop the fan with a rolled up newspaper if it stops or at least tries to again its okay, if it shreds the paper its a duffer.

Most 200 owners dont run the viscous fan at all, having a elec fan as a back up.

Thanks, I'll keep an eye on the belt, my fan is rusted up so may look into the electric option and add it to the list of jobs.

The cheapo universal fans off of ebay are just fine, 16 inch is the ideal size about 35 quid, with the viscous and the shroud removed it makes engine access a doddle, worth doing for that alone.