Aha! A completely different perspective. We've been told about another oil company to use which should be ok and if not Mr Snoodles found something on you tube about self cleaning which should keep our service levels same.
....This is what it describes (sort of) Fifixx ....switch Aga off, undue little screw on pipe at front, lift out the tray with burners on, take off the various concentric shells, undo where the pipe attaches to burner and then you put a drill through it t clear out the sulphur, blow through. Get rid of carbon on the concentric bits. We are going to give it a go in a few months...if we can get a handle on it ourselves, it will save us a fair bit. £80 a pop!
Reading that article I guess we've been lucky....this is eh first time we've had this problem.