you should change chain and rear cassette at the same time. Probability is they're both worn out and a fresh chain needs fresh cogs or you'll have no improvement. Take it to a nice lbs if poss and avoid the Evans. You'll need to know what style of cassette you need if you're going to do it yourself. Google sheldon brown for all your answers, factor in tools to get the job done into your cost if you decide diy. Buy your bits from a local and they might even fit it for free, if not cheap. I'd expect around £50 cost for a bb, chain and cassette parts.
Edit - budget parts, I should add.
you should change chain and rear cassette at the same time. Probability is they're both worn out and a fresh chain needs fresh cogs or you'll have no improvement. Take it to a nice lbs if poss and avoid the Evans. You'll need to know what style of cassette you need if you're going to do it yourself. Google sheldon brown for all your answers, factor in tools to get the job done into your cost if you decide diy. Buy your bits from a local and they might even fit it for free, if not cheap. I'd expect around £50 cost for a bb, chain and cassette parts.
Edit - budget parts, I should add.