Hi Nicola, and poor Tilly.
Do you have her insured? If yes, and the x-rays don't show anything but you're still having problems you could ask about getting some cross-sectional imaging done which would look at soft tissue as well as bone structures (CT or MR). I'm not sure but guess that the Glasgow vet school will have some scanners. I also once had a horse taken in to the vet school for some radionucleotide imaging for a recurrent and unexplained lameness (can't remember exactly what the scan was called).
Meantime, I'd be disciplined about following the rest instructions. It could be an injury that is just never quite healing before she pings it again. And once the 3 weeks is up be careful to build the exercise back up slowly, and don't let her go haring about on the first day.
There's a horse / canine physio down my way (Ayrshire) who maybe wouldn't be too far for you if it comes to that. I've copied their website address below in case it helps.
www.animalphysioscotland.freeserve.co.uk