Well today I managed to take the car over to Compressortech for a quick diagnosis. The service there was excellent and a technician got onto the car immediately and found the centre part of the pulley is what is rattling. They did explain the finer details but I must admit I have forgotten what he said now!!! But, it sounds like there is not real risk of the belt coming off. However, although it can be fixed, the labour time and cost is similar to replacing the whole compressor, and there is no guarantee that the compressor would fail after having this fixed anyway (even though it is functioning OK despite the noise!!)
I wasn't charged for the diagnosis, but the cost of a remanufactured a/c compressor (they remanufacture compressors themselves) fitted, is around about £450 including the VAT. So it's still a fair bit, but it is not a figure that is worse than what I was expecting. So, I think I will be booking the Aero in in a couple of weeks.