Hi,
I have just heard back from the garage, it was just the bulb, they have changed it and it's now working fine. They're just going to investigate the knocking noise now. It will be interesting to see if they sort it or whether they "can't hear it"....being a 2nd hand car under warranty it wouldn't surprise me if they try to get out of it, but we shall see.
I strongly suspect they won't find the drone noise, but it's going to another place in under 2 weeks and they will sort it I'm sure, but it'd be nice to not need to pay for it!