putting xx£ in and counting miles is not really the most scientific way to check your fuel consumption, especially with a mk3 fuel gauge. fill up completely and mark mileage... then drive until it's fairly empty and fill up. from that calculate your l/km or l/mi.
who told you the tb doesn't need adaptation? if it is a 60hp from 97 you will need adapt it. and no disconnecting the battery is not a tba... also where did you get that 'new' throttle from? could be having broken gears in idle stabilisation as well.
yours doesnt have a relay 30 cos it doesn't need one.
would suggest a full diagnosis with vcds - that's not just checking fault codes but also if all parameters the engine gets make sense, i.e. temperatures from all temp senders, voltages from lambda,.....