There's only one shop in Sevenoaks (I think it's called Bike Warehouse) and I'm not sure I'd trust them to do servicing. I have to say that I don't have any evidence for my opinion, it's just a feeling I've got from going in there a couple of times - the shop just doesn't feel 'right'.
Wildside (aka Wildprice) in Tunbridge Wells is an option, but expect to pay through the nose and be treated like a pleb.
You could head up to Bromley for Bromley Bike Co or Bigfoot Bikes, but my preference would be for Petra Cycles in Oxted. Just down the A25, parking outside and a good shop - like a proper LBS should be.