SamF: Ok, so I'm a glutton for punishment, but my curiosity got the better of me and I tried changing RFI back to 1 on my connection. There was an immediate increase in FECs, but no measurable increase in CRCs. Swapping around different RFI levels made no measurable difference and I'm now back where I started with RFI at 5.
I decided to try changing RFI again on mine to see if it still makes a difference.
CRCPM error rate at 10am this morning was 0.05 (and it has been since last weekend) This is with RFI set at 2 (one left of max performance)
I put it back to RFI 1 at 10am (did the usual unplug line, change setting, soft restart, power cycle, check setting, plug line back in) - errors went straight up to 0.9 CRCPM. 3 hours later at 13:00 they had settled on 0.8. So an order of magnitude greater than they were.
So I put RFI back to 2 at 13:00 and now an hour later they're back at 0.06. Seems quite consistent on my line but either my line is different or my fritzbox is behaving differently. Firmware is 99.05.22 FWIW.