if the gearbox is removed is there any way to accurately check the t.d.c. position of the engine front strobe timing plate by the flex plate position

 

No, Brinny, the flex plate has no position marks and can be fitted in any rotational position. The best way to ensure the timing plate is accurate is to remove the plugs and turn the engine slowly clockwise only (looking from the front) and place a probe into A 1 plug hole and mark carefully when it stops rising, and mark carefully again when it starts falling, and TDC is halfway between these points. A helper is best and do it a few times to be sure. The piston moves hardly at all in the last few degrees before and after TDC, hence the two marks. If you wish to set up the actual spark timing (as in setting the firing of the ignition at A1 at the correct moment) ensure you are on the compression stroke by feeling the pressure build with a thumb over the hole; then, of course, it is two crankshaft turns to the next firing-stroke TDC.
The probe could be a piece of hard wire sliding up and down a hole drilled in a cork, for instance. The cork gets pushed into the plughole a touch, and it holds the wire from slopping about in the hole.
Thus method ensures any timing chain stretch is accounted for; BUT if there is any such stretch (pretty unlikely I would say), the timing between A bank cam and B bank cam will be slightly affected, so the perfectionist would adjust for that on the B bank cam-sprocket adjusters! Personally unless a full rebuild is being done, on a road car engine with under 200,000 miles of normal use, I think that would be a waste of time.