As promied, I have had a quick trawl of the net to see if I could find any suitable tile edge trims that could be used with our bathroom cladding.
I haven't been able to find that many sites selling this type of product but I have found one that sells online - Lifetile
They have some 7mm and 9.5mm trims which could be used although only the smaller trims are available in a range of colours.
Even though these trims are 1mm smaller than a lot of our Grosfillex cladding they could be used by cutting off the tab and using the trim as a straightforward quadrant.
I will have another look around if I can locate marble tile edge trims - I've definately seen them somewhere!