Ok thanks Si! I just figured out that the fan was not working this morning and I'm not too comfortable about taking the fan apart and repairing it (the fan motor seems to have gone).
For other peoples information, if you get just a humming noce and no air from the flue then your fan is probably faulty like mine was.. it just wasn't turning!
Its cost me £100 from keeptheheaton.com to replace the fan, (although the local shops quoted me £150) and I've just finished the job and the boiler is happy now :-)
Thanks for your help Si; much appreciated!
Paras Shah
August 2006