So I ordered a Mann filter online - it appeared to be the correct filter,but when I removed the old (very rusty) filter it was obvious that the new filter was a slightly larger diameter LOL.So I refitted the old one - only to find it had developed a pinhole in the rustiest part of the body and kept dripping fuel

So have temp fitted the new filter and will have to drive 35 miles tomorrow to pick up (hopefully) the correct filter LOL (the new filter does not fit in the mounting bracket).
The point of this post is that sometimes components need replacing on calender time as well as mileage.
Was it the original fuel filter still fitted to our GV ? - probably - it is a Mikuni.