It all depends what the 'full service' includes imo.
Quite often a service is just engine oil and filter, air filter, sparks.
A service can include various fluids such as engine collant, air-con service, new belts, fuel filter, gearbox oil.
If you buy the parts yourself the engine oil is £15-25, oil filter £5-£10, air filter £5, sparks £10.
All depends how new the car is really and if you think the full service history is worth it.
An indy garage will be cheaper I would think I reckon you should get a quote off a few.