I have to take my MBP into the Apple store in Cardiff on Friday to get fixed (sounds like the bearings on one of the fans have gone), does anyone know how long (approx.) it'll be before I get it back?

Depends on the work load?...I know the Apple retailer in Bath have in-house Apple certified engineers and a small fix like a fan could be done while I wait or could take two weeks while it waits in a queue...

As to whether Apple stores have in-house engineers or not is another question...

Yup. They are generally pretty good at giving an indication of how long something will take (and I have the luxury of being able to go whenever they call me to say there's a bit of time that's not too busy).