Many thanks, guys, for all the replies and suggestions (especially the
yummy recipe!). Never anticipated such a huge response!

Just to address a couple of points:
a) I'm not a total novice to air-guns, just a little "rusty". Used to use
one to shoot rats and other vermin on my uncle's farm when a teenager, many
years ago!

b) Not looking to exterminate the entire wildlife population to protect my
plants. I'm looking to get rid of the pigeons and squirrels who are taking
up all the "goodies" I leave out for the blackbirds, sparrows, tits and
other various, very welcome and non verminous wildbirds that visit. I have a
huge wood at the bottom of my garden full of said pigeons and squirrels and
would rather them stay there than start infesting my garden and eventually
my house! If you've ever had a squirrel in your roofspace you know what I'm
talking about!

Clive in Kent