Thanks for all your answers, I'll try to answer the questions in order.
1) I'll be honest and say that I don't know how to clean the barrel. I did try using a Qtip and gently wiping the inside of the barrel. What is the best way to clean the barrel?
2) I've had a look at where the BB goes into the barrel and I don't see that the rubber is dry, cracked, torn or broken. I did observe that there is a lump in the rubber (I presume is the hop-up ball bearing) positioned at approx 8 o'clock.
3) I'm using new BBs each time (was advised not to recycle BBs), they're unbranded and pure white.
4) In terms of accuracy, I was shooting 4" targets easily from 21' Now I'll miss 4-5 shots out of the whole magazine. I tried shooting a plastic milk bottle (half gallon) and I get the same thing, about 4-5 shots go astray.