In case it is of interest here I have had no joy sourcing my filter element. Got the following from a reply from Lee Mitchell, who runs the WDB40.com site:
"The problem you have is that the Vokes and british filters are different filters and as a consequence so is the filter element.
Another problem is the Vokes elements are unavailable, but the british filter ones are, usually about £10
I am still looking to find something that will do the job in a Vokes filter so will keep you posted if I sourse anything."
Will have to look to modding the nearest modern equivalent I think.
Mike