Do you go over to the cage every time they chew at the bars?
I had one rat who had me trained to do just that. Princess particularly enjoyed doing it right as I was going to bed, or right after I'd turned off the light.
::CHEWCHEWCHEW::
I'd get out of bed, turn on the light, take her out of the cage, tell her "no chewie the bars!" and put her back in, turn out the light, back into bed.
::CHEWCHEWCHEW::
I tried just ignoring it, but extinguishing that kind of behavior can take hours and days of not reacting to a very irritating sound (especially irritating when you're trying to sleep!). Finally I got a water spritzer and put it on its stream setting, turned off the lights, and crept to a couch a few feet away from where she was about to
::CHEWCHEWsquirt!::
Didn't hurt her at all, just got her coat a bit wet, so she cleaned herself off and probably wondered from whence it came. Repeat this a few times, and she stopped chewing the bars entirely - she wasn't getting any interaction out of me by doing it, and there was just a hint of a negative outcome whenever she chewed the bars.
Your rat could also theoretically be chewing to wear down her teeth or because she finds it amusing in and of itself - you can buy sticks of applewood or various hardwoods at the store and hang them up in the cage. Some rats will chew those, but others won't - it's a very personal decision.
A far more universally beloved chewtoy though is actually the Ecotrition Snak Shack Hide-away (though I just noticed you live in Manchester and I've no idea if you can get these in England):
http://www.petsmart.com/product/index.j ... lInCA%2FNo
Ecotrition Snak Shack Log:
http://www.petsmart.com/product/index.j ... lInCA%2FNo
In both cases, don't get the smaller sizes as they don't fit rats, but the large sizes can be eaten for months on end. They're made out of alfalfa and honey that are pressed together into the mold until it's very hard and unbreakable - but VERY chewable for rat teeth and probably tasty to boot. I've always had a tiki hut or log (and sometimes) both in my cages and it's really funny to see where each rat chooses to chew - some will chew protrusions, while others will just widen the door.