Christ, I used to get 34 average from mine and 'er indoors gets 30 from here Ibiza Gti which has the same engine. The ibiza does 30 mph stuff and only 5 miles each way so that is understandable really.
As for the bulb change, if its the same as the Ibiza (similar clocks??) it two small screws underneath the top edge of the pod the clocks sit in. Undo them and pull the top edge of the pod towards you whilst it rotates about its bottom edge, bulbs are on the rear face and twist out.
Personally, it sounds like it needs the timing setting up, might be worth a good full service (plugs, leads, rotor arm, dizzy cap, air filter, oil and filter & fuel filter), stick a tank of optimax through it (cleans the fuel lines etc) spray some carb cleaner into the inlet and then take it from there. If unsure, a local independant will hook it up to their diagnostics for a few bob and see if theres owt desperate wrong.