OK, so I couldn't see anything similar to that in the roof space near the cylinder, so I went hunting in the airing cupboard below and saw this:
without touching the blue valve, I turned the one at the top anti-clockwise for about 2 seconds during which I heard the rush of water.
went down...
Only been in this house few months, so haven't got 'acquainted' with our boiler yet. Back from holiday two months ago and spanner was showing and 0.4 pressure. Put on heating to test it and it fired up OK. Relief all round.
Two days ago, we go dreaded F22 error and nothing was working, heating...