The official government website isn't quite as prescriptive as Ethel's answer. Quote:
"Can I use a forward facing child seat or booster in a seat fitted with an active air-bag?
See what the car handbook says about children in seats with frontal air-bags. This can vary from car to car. We strongly recommend that advice should be followed because air-bags are powerful safety devices that must be treated with respect. Children, and adults for that matter, should not sit close to an active frontal air-bag. We suggest that the car seat should be as far back as possible from an air-bag."
Source:
http://www.thinkroadsafety.gov.uk/campaigns/ch ildcarseats/childcarseats.htm#18
Chris