update of problem
I took my engine into my local mariner dealer who after 10 mins on test and it hadnt stalled, rang me and told me that the 2.5 and 3.5 hp Mariners have a "known fault" causing them to stall when the fuel in the tank gets to half or below or if the engine isnt perfectly level. Appraently the vibration of the prop causes the half filled fuel tank to allow air to be sucked in and hence it stalls!! nothing you can do about it he said.. I couldnt beleive what I was hearing. apparently he said all Mariner technicians know about it!!??? Seems a bit odd from a reputable manufacturer.
Anyway, he rang back 20 minutes later and told me he had just seen the engine stall and there is no spark. He has checked the basics but not found the issue but will investigate fully on Monday!
I more pleased now 1) that he has seen it stall on a level test bed with full tank of fuel! and 2) he has seen there is no spark.
anyway thought i'd send you an update and will update further when I know its fixed