I got a reply this morning. Clearly they only have one representative who has been on holiday.
I also wish they would stop telling us to play with the stability adjustment - setting target S/N should be left to the DLM. This is the only device I know with stability adjustment (maybe for a known reason).
I guess that's Paul Lee they are referring to.
They sent me those settings also, but to be honest they shouldn't be needed to make a connection stable. The ZyXel box I've been using recently doesn't need them and I get far less errors, plus I now have more confidence in it holding the connection with the latest firmware. I'm tempted to try returning the ASUS to PC World as unfit for purpose.
I stuck the ASUS back on-line yesterday for 12 hours and despite maintaining a connection all day DLM smacked me down by 2mbps, so the 2mbps I gained last week with the ZyXel was swiftly taken from me as punishment. The ZyXel is back in place so hopefully by Wednesday I'll be bumped up again.