Low Presure on Valliant EcoTec Pro boiler

Hi Everyone, I thought I would ask if anyone knew how to fill the above boiler.
My water pressure is 0.6 and I know it should be filled to get it back up but can't work out how to do it.
I know where my filling loop is, and have located the two black toggles, and that should be it, but I cannot move the left toggle at all - I know they are both meant to turn.
The two toggles do differ slightly in shape, so I'm wondering if there is some sort of safety catch I am missing? Unfortunately there was no information left when I moved in.
any help would be greatly appreciated!!!
Thanks.

If you have not bothered to regularly keep your boiler topped up then the shaft may have seized.

Usually if the plastic knob can be removed then the shaft can be turned by a tight fitting adjustable spanner.

Tony

Oh, thank you Tony for your reply.

Its my first property and I didn't know I should be doing something with it regularly!!

The last time someone looked at it was Dec 09 when I moved in. Could it have seized since then? I did hope it was something I could fix, and in theory it looks like it should be, but not sure I will manage to if its seized.