Page 6 of the manual
clicky shows the filling loop .
2 taps need opening either end of the loop
WRAS regs include the fitting of a check valve to stop boiler water going back into drinking water, this could be stuck shut also .
if you suffer low water pressure you may not get the required 1 to 1.5 bar required
(sometimes filling very late at night when local pressure is highest ,due to low demand, works)
that said ,just read your reply about it never being serviced ,
might be wise to let the charity have a look if it's no cost to you
condensing boilers do need regular service ,adjustment to work efficiently
10 years of no service will probably mean the boiler is ****ed anyway now
so ignore me
so let the Gas Safe "engineers" deal with it ,especially if it's FREE