Can't re-pressurise Valliant combi boiler - HELP!!!

Morning all, hope you've had a nice hot shower this moring...

I have a Vaillant combi boiler (no model number on unit or manual in house), but from looking on website looks like older/basic ecotec model or similar.

The system stopped giving hot water last night and comes up with error code F28 (failure to ignite?), and there is zero water pressure according to dial on panel. This happened last year when first turned heating system on after being off all summer, called their 50p/min tech line and found out how to repressurise system (2 valves under boiler), problem solved. Unfortunately techservice closed til monday, so have tried repressurising system again but nothing happens, i.e. no sound of water flowing when open valves and no change in pressure, so unsurprisingly boiler won't fire.

Please can anyone advise

a) if I'm correctly attempting to repressurise - right hand screw valve turned to 3 o'clock (also tried both)

b) If there is some sort of air lock in system (recently stopped using CH system although been turned on/off few times - may be relevant) - we have quite big 3 storey house with loads rads and not sure how would go about bleeding system if this is necc?

Any help greatly appreciated

Keep fumbling with the filling mechanism - it should work- ie re-pressurise, then fire up.
You'll hear it & it won't harm owt

You need to make sure both valves are open at the same time, that's all
though vaillant is not a bad boiler, their after sales is amongst the worst, if not THE worst.

Cheers for the tips, thought I'd tried every combination of valve openings, but not simultaneously... Does it matter what state boiler is in while doing this, i.e. on/off, hot water or heating any particular setting?

I do one of 2 things if possible:
every rad fully hot with boiler at max; bring pressure up to 2.5
Whole system fully cold; between 1 and 1.3