I thought I'd updated this, but clearly not!
Not much to update but I'm not ignoring this, I'm just not doing a lot of miles at the moment!
I've found the top return pipe was blocked, so I unblocked it (by removing the restrictor that was in place and caked in ), that seems to help a lot as the temperature keeps at a constant 80ish now. I've only done 160 miles since that was done though, and of that only 1 long trip (60 miles each way), so it's a bit early to tell still. I'll update again once I've managed to take it for a longer run, which I know would have been an issue in the past!