Stephen,
If the manufacturer cannot help you, and your local camping / caravan shops don't stock these, then it's time to look through the on-line pages of electrical and electronics suppliers - like Maplin, RS Compoenents and Farnell:
Maplin - http://www.maplin.co.uk/family.aspx?menu=31&MenuName=Fuses+%26amp%3b+Circuit+Breakers&worldid=3&doy=23m5
Farnell - http://uk.farnell.com/ then do a search for "Fuse Holders"
Robert