I know it fails the under £100 requirement, but what about the Billion 8900X R3? (just over £140 on Amazon, 1 left in stock...)
It's basically the same as my 8900AX-2400 but without the wi-fi, and the 8900AX is rock-solid - current uptime is 208 days (and that was only because I powered it off to tidy up cables) and it never, ever loses sync even on my long, crappy error-prone line. I've tried a Zyxel 8924 and it was no more (or less) reliable, but did have some annoying DNS-related "features" that work fine on the Billion.