Just had a garage replace the oil cooler which had been leaking. When I collected the car they initially said everything was fine, then when he started it to drive it from the workshop, the serpentine belt was squeaking and rattling like . Is this likely to be something they have got wrong with the repair as there were no issues with the belt when I dropped the car off?

Removal of the oil cooler has nothing to do with any of the belt(s). It's always possible they may have spilled something on it though. The older I get, the more I realise that people confuse wrinkles for wisdom

Ok thanks. I’ll replace the belt.

Check the tensioner and pulleys for any play, if you need to replace those then it’s worth doing alternator and water pump at the same time. Can get expensive very quick but it’ll save stripping it down again.