My long day turned into quite a short one, after about 2 hours of messing about trying to measure the horizontal cylinder valve lift it was obvious that my spurious results were being caused by the DTI not following the movement of the rocker arm. Lazy spring, a slightly bent actuating spindle or possibly something internal to the mechanism catching, whatever, I decided a new one was required.
I had to wait two days and then, of course, it would not work with my Mk2 holding bracket so a new one was fabricated.
Then I got a cold and stayed indoors for the next day.
Back on it again next week and I think I have a better way of setting the valve timing than relying on guessing the position of non-existent marks on the inner cam belt cover. Namely, first setting the adjustable pulleys in the middle of their adjustment, putting the crankshaft on the right stroke for the opening of the inlet valve, then positioning the engine at the point where the inlet cam is due to open and rotating the camshaft to the point where the DTI registers the first movement then aligning the camshaft pulley keyway with the belt on it and pushing the belt and pulley on.
Should be a breeze with the possible exception that foolishly I loctited everything on first assembly in the belief that I would never be more than a few degrees out (it is obvious that I know enough to be dangerous but nowhere near enough to be good) and if I was it could be easily accommodated by the adjustable pulley