I have several gas 'plinkers' pellet, BB and dual - all of which are quiet, but my favourite is a Zoraki HP-01 (sometimes called Webley Alecto) which I've fitted with a Webley moderator.
The gun uses a pump action to build up compression - so there's no gas cartridge costs.
I've fitted it with a modular stock which turned it into a rifle.
Add a red dot sight and I have accuracy.
It's very powerful for an air pistol – but quiet.
Having said all that you could probably pick up a cheap PCP rifle for the same money.
Find a local, reputable gun shop and take their advice - they want you to keep coming back so should give you good advice - and you can try a gun out first.
thanks for the advice. I've got a fairly big garden with plenty of room for safe shooting but has to be quiet. I will have a look at the Zoraki. Thanks again