Worcester heatslave 9.24 - can't re-pressurise!?

Hi - just moved into a new house, with functioning boiler (worcester heatslave 9.24). Bled the radiators (lots of air) and now no hot water. Took the front cover off and sure enough the pressure guage is on zero. Problem is, I can't find an obvious tap/valve to re-pressurise the system! There are a couple of taps on the pipework below the boiler, but all already fully open. I can't find any screwdriver type valves. Is there a particular trick with this type of boiler I'm missing or just not looking hard enough!? Any tips much appreciated!

Thanks, Ian