In article , "Ronald
Fraser" says...

Is there a workaround that would enable me to pressurise my Combi to
1bar when the mains water pressure is only 0.75bar ?
The combi is working and when heating the pressure rises to 0.9bar.
I am concerned that the combi should be operating at the recommended
pressure.
Some have suggested attaching a 1.5bar shower pump ( with attached
water tank). Is there an alternative e.g. water pump that would serve
to bring the combi up to 1bar purely for commissioning purposes.
There is adequate DHW water flow.

You could use a car tyre pump (not an electric one obviously), a
welly and a bowl of water. Just attach a schrader valve to the
filling hose to act as a non-return valve, and fill everything with
water before you start pumping or you'll introduce air into the
system which will need to be bled, losing pressure again.