I understand guns are not quite as easy to come by in the UK. What you need is hot-pepper treatment. They make a spray to treat bird seed. Birds can't taste the hot pepper, but squirels don't like it. The problem is that the spray looses it's potency quickly. You could also try your own pepper concoction. I'd put some cayenne in the bird food and see if that works. (Side note, some people add cayenne to boost egg production- not sure if it really works for that.)