Thank you, Ed, for this valuable intervention. I have posted it on the other thread, too, so readers can benefit from your insights. It is as well that I raised the advice I received from the gunsmith here first and did not act on it.
I have checked the screws to the plate alongside the loading tap and they are tight.
I cleaned the barrel with a .22 cleaning rod sometime ago (with the loading tap up).
Yes, I have tried various pellets and still have some old Eley Wasps. It read 479 ft/sec today.
My practical abilities and lack of tools are such that I will not strip the weapon although I could open the loading tap, as that is quite simple.
Thank you for your advice - much appreciated.
A