If you can change the oil and oil filter yourself, do it and save some money. Infact, with the money saved, you can invest in some good castrol oil
You may want to inspect the brakes - if they havn't been done for a while, and you've ridden the bike through winter, a good strip, clean, regrease and reassemble will be a good idea. Clean and lubricate the chain if you havn't done that for a while, and clean around the clutch work-drive area.
Give spannerman a ring for 4000 mile service advice (he's based in sunbury, near hampton court); the oil and the oil filter are the important bits, but you might like to do other work while you have tools out and are covered in bike-related oily muck!