Hi and welcome to the forum. Has the booster heater been working correctly in the past? ie, loud roaring sound and no visible smoke from small exhaust after a few minutes running, as you have a 55 reg you will most likely have the later ceramic plug with wires attached which are available from dealers or there are outlets on ebay, (dieselheaters4u and heaterpartsdirect). Would advise cleaning combustion chamber and checking air inlet pipe and exhaust for blockages. Instructions with pictures for servicing in the FAQ's of the technical section http://www.fordgalaxy.org.uk/ford/index.php?showtopic=3623&view=findpost&p=24940 go to page 6 then scroll down to thread 80 onwards.