Thanks Richardthe3rd and JonoH! I tried pliers but the plastic started to disintegrate so I thought better of it.
In the end I asked my ex-husband to have a look and he turned it on like, well, a tap! So it was just too tight for me. Damn those double-X chromosomes!
Thanks to everyone for...
Thanks Tibbot and Whitespirit66! That was very helpful.
I went down for another look (for some reason the boiler is in the basement) and it turns out the screw is rusty and slightly stripped (no idea why; I haven't ever touched it). So it's not a DIY project any more; I have to get someone...
Thanks so much for your reply!
It's a Sabre HE Plus boiler. I've wiggled and jiggled and tickled the key or tap or whatever it is, but it won't move at all in any direction.
I'll see if I can locate the drain valve...
So my heating and hot water haven't come on today and I'm very cold! I checked my combi boiler and it has no pressure. I read up on what to do, and it all looked so easy on youtube. I saw that my filling loop was left attached by the man who installed the boiler, so it seemed to be a case of...