Before you had the BT Openreach out your line attenuation was 50,now it is 59.
I do not know if it because the router is show a wrong stat but u might want to try your other router again to see if its showing 59.
That is a big jump up and your speed will suffer because of this.

Ah well. At least it's rock solid now, no more than a .1 variation in SNR since it was put in. On the old line it was up and down like a madam's wotsits.

Hey I never noticed that. I'll try with the belkin back on again tonight. Well spotted. It might well be router reporting differences but it's certainly worth checking.