when our old one was oil (basic burner,gravity no electrics ) ...... i always serviced it myself using the correct sized drill bit (13mm ) turned by hand in the main oil hole to clean out carbon (clogging it up and disrupted oil flow) that we used to get a lot a it was mostly on a low tick over setting.
....and a bought and fitted new 'wick '....
...... all very straight forward to do,
just got watch what the service engineer does first time .....
+1 bought a roll of wick off ebay (enough for 10 years ) cut the cutouts for the oilways with scissors, very easy to do. Did have a problem a few years ago, so got a so called expert on Rayburn's out, decking clueless he was ! cost an arm and a leg, and it was no better . Trawled the internet and figured out that it was the slit in the oil control valve that was gummed up. Gently cleaned it and the Rayburn was working like a new one in no time