I'd get the installer back - clearly down to him ;-)

[Ducks! :-)]

Well, Worcester-Bosch got back to me. From the horse's mouth:

Thank you for your recent enquiry. I have found that by using
a small flat blade screwdriver inserted in to the slot on the
button, and a plastic rule as a contact point of leverage, I
was able to remove the button successfully. However, I do accept
your comment that it does seem to be very tight.

I tried it (with a chisel instead of a screwdriver and a piece of offcut T&E
rather than a plastic rule). It worked easily with hardly any force. I think
that the lever idea works not because of the extra force that could have
been applied, but because it applies the force at the specific angle if
seems to require.

It's too early to say what the recovery performance is now. I haven't even
had time to ensure that the flow temp gets above 75C, as I only had 2
minutes to spare and it was during a 3 minute anti cycling cut out.

Christian.