Just thought I'd update this in case anyone finds it on a search.

I limped along with my original PCB during the recent cold weather. I had tried prodding it - a plastic baby spoon worked well! - and found two of the relays clicked when nudged, so a bit of gentle 'packing' helped them work..

I decided to bite the bullet and get another refurb board. Low and behold it worked fine.

Quite why the first replacement didn't work at all I don't know. I've read of this before with Suprimas but hadn't thought of it my case, I assumed the fault was elsewhere. I guess there's something about the parameters the board checks on power up and maybe some are more fussy than others.