I just got to the beginning of Part Three on my reread last night, and noticed something that I had missed on my first read, when I didn't yet know whose photo Polly had stolen from Hunsdon House. (Though maybe I'm just obtuse and DWJ expected me to have figured out about the photo already, heh.)

Anyway: It must have been Polly who gave that other string quartet the flu!

She was at Reg and Joanna's, and couldn't sleep, so she got out the stolen photo and fell asleep holding it. And then, lo and behold, those poor unknown musicians are felled by the flu, and the Dumas Quartet stays in town one day longer than planned, and they are there when Polly needs help.

I suspect this is why Mr. Leroy found Polly in Bristol, too, that he knew that some magic had been worked there on Polly's behalf.