Thanks. There is a very small nut on the reverse, which you can't even see properly because there's so little access (camera couldn't manage auto-focus on it either at night) and it's painted over. God knows what size spanner is supposed to fit as it feels like a completely random shape - I'll try to get a photo tomorrow and figure it out from that.
If it's a normal bleed valve it'll be a square cross-section, not a normal hexagonal nut of any size.