I am publishing this post just in case anyone is/was a bit confused regarding the fuse at the top of the Alde boiler under the black plastic cover.
I thought I would order some off E-Bay as they were cheap enough, £2.75 for a pack of ten.
When they arrived I was concerned they they were labelled at 250V as opposed to 12V which is what I thought they should be.
I checked with Alde and it is OK at 250V.
The specifications are:
20mm x 5mm
I got them from here:-
of course there are other outlets.