OK, not overnight, but in the middle of the afternoon.
Coupled with losing about 7Mb/s from my sync, my latency went from approx 10ms to 20ms during a resync. I'm on an 80/20 Plusnet line, and usually get 79 down and 20 up. But I've dropped to around 77 for some unexplained reason.
I'm more confused about why this would happen than wanting an immediate fix.
I'm using a TP-link router, which 'helpfully' tells me that my Max Rate is 87133, but my current rate is 73006. I'm confused!