worcester danesmoor oil boiler 32/50 conventional flue - lockout... ?

Boiler only about 9 years old - for last 2 months has been locking out all the time, changed nozzle, been serviced, changed photocell (although doesn't seem to be anything wrong with it) finally found that burner is getting clogged up with sooty tar but why now??? Did have new oil delivery a few weeks before this started but isn't all oil meant to be same sourced as a commodity! now having to clean out the burner every week as it locks out again. any ideas out there why this has started happening gratefully received!