Thanks guys
Good new is there was very little on the filter element 2 or 3 very small bits of metal no more and maybe less than last time when I stripped and inspected the filter. I did measure the canister depth last time too but got something wrong .
After reading Snowbum and Anton Largiader yet again to make sure I get it right this time I've come to this simple conclusion
The O ring must be under enough but not too much compression in the range 0.2 - 0.8mm
and
you must measure your own gasket and O ring thickness which may differ from Antons Figures.
My gasket was thicker and O ring thinner than I had assumed.
I had 3 new filters on the shelf 2 of which are the same make and they are all different lengths varying by upto 2mm.
By trial assembly without the O ring in place I have  concluded that the ideal length including end seals is 129.5 mm but this would be 0.55 mm less if the gasket was not used! This really does suggest that OEM filters maybe best.
Here's my simple speadsheet for calculating the O ring compression.
My canister depth is 3.0 mm so I do need the gasket and have fitted it his time.
