British Gas will want to install and charge for a new boiler.
If the fan fails to run or runs poorly then the boiler will not light.
My idea is that you sack British Gas and engage the services of a reliable and trustworthy independent boiler engineer.
My Potterton boiler was installed here in 1998 - I am on my last legs but the boiler works fine.
I do not understand why or how your fan can possibly be full of water - the fan moves air not water - if the fan had water in it it would be dripping out over the floor.
Good Luck...
January 2016