I just hope that they didn't say "We charge for a minimum of one hour's labour, at £80 plus VAT, so that will be £96, please - oh, and enjoy your soup!"
Having spent many hours recently rearranging everything in the boot (in case there was something in there rattling) and going for a test drive, then taking everything out of the boot and going for a test drive, then tightening up the spare wheel and going for a test drive, . . . etc . . . etc . . . etc, I finally went down to my local garage and asked if they'd got any ideas.
The guy said "I can't look at it right now but bring it in any time tomorrow and I'll see if I can sort it out. It will probably turn out to be the lock that's rattling though; it nearly always is".
So I took it away and found out that, because it was badly in need of lubrication, the catch on the lock wasn't fully engaging with the bit that it was meant to meet up with, causing the rattling. A quick squirt of WD40 was all that was needed to fix the problem! (I dropped 4 cans of Stella into the garage later on though, to thank the guy for his tip).