Thanks for your reply.

We have a filling loop, usually when we turn it on, there is the sound
of rushing water, and the pressure goes back up into the ideal range.
At the moment when we operate the filling loop, nothing happens at
all, and there is no sound of rushing water.

We were wondering if this is symptomatic of a broken filling loop, or
just a side effect of something else.

We have a car tyre pump with gauge, but we're too scared to attach it
to the expansion vessel value, in case it over-inflates the vessel
and ruins it. Also, the gauge on the car tyre gauge goes up to 250 psi
- much more than the 7 psi needed for the expansion vessel, so
it's not a very good gauge of how much pressure is in the vessel -
attaching the pump to the vessel valve barely moves the needle.

Also, is using a bicycle pump a practical way to repressurize the
expansion vessel? We pumped away for a while, but had no effect when
we
switched the boiler back on??? How much pumping should be necessary?