Wee update.
Set a cap at 41000/15000 which resulted in SNRM of 8.2 and the FEC numbers reduced. However, something happened either Saturday or Sunday afternoon and the modem lost sync briefly, retrain reason 2, and when it came back up it was at the full line speed for the line with interleaving. According to the web interface on the HG612, it hadn't restarted. As it was running full line speed, the FEC numbers were increasing again at an unhealthy looking rate.
I took the opportunity to replace the MK3 faceplate with a new MK3 faceplate and I also removed a Netgear Orbi from the area as maybe its PSU is emitting a lot of noise, something I'd overlooked when I was thinking about what had changed around the start of the year that could have moved the line onto interleaving in the first place.
Running at the full 51Mbps for the line with interleaving, the FEC errors have dropped by about 80% - 85% of what they were previously. Looking at the stats, sometimes will have 0 FEC for several minutes then get a burst of a couple of thousand, then all quiet again and sometimes single / double digit FEC errors. But that 80/85% reduction in FEC seems to be a step in the right direction.
With little / no CRC/ES/SES hopefully reducing the FEC by this amount might tease the line back to fastpath (I doubt it, but can hope)!