Thanks all for your responses...
I bought the car from VW and therefore hope the wheel bearings aren't shot as this should have been checked...and thinking back it has made this noise ever since I bought it.
I believe it's the tyres and as has been suggested above, probably the rears (Bridgestones), although tyre wear doesn't look too bad i.e. they still have a fair bit of meat on them.
Thing is, how do I stop it happening again?
Russ