Crossbow Tinkering

I've grafted 20mm of extra length onto the end of the track, pretty much what I'd sawn off when working with the wooden prods. This gives extra draw weight and draw length which will give a faster flatter trajectory. Whilst doing this it became apparent why I was shooting all over the place, just about every screw and bolt was loose, presumably shaken loose by the recoil.
I got it all back together nice and secure, I also reserved the centre serving which had worn through, well the to be more accurate, the second layer had worn through, but the first layer was still sound.

Taking my brothers advice, I got myself set up in a seated position with the crossbow supported on a tightly rolled old duvet resting on my workmate at 10 yards range, working on the premise that I wanted to be aiming 3 dots low at 10 yards to give me the cross hairs on target at about 20 or so yards.
1st shot, steady as a rock 3" low 4" left. I wanted to adjust to the right and it became obvious that I had too many sources of error/adjustment. The scope rail is screwed into the wood block that covers the trigger mechanism/latch with 2 wood screws (plenty of play there) and that block is screwed into the stock with 2 off 6mm bolts which also have slop/adjustment. I slackened the bolts and tweaked it.
2nd shot, same height (good) 4" right. Adjust again...
3rd shot, same height (good) 1/2" left, that'll do for now.
Looking at the height, I obviously don't need to be aiming 3 dots low, as it's going low... try one dot low.
4th shot 1/2" high, 1/2" left. Well that's pretty good, but want to be aiming 3 dots low at 10 yards to give a good point on range. So I added a 2nd shim of thin tin plate (the first had been added before last weekends testing). The tin plate is 0.01" thick. Then I tried again aiming 3 dots low.
5th shot, about 1" high and 3" left ...
Well it's obvious I can shim out the height successfully, but the left right adjustment is too prone to being knocked out of kilter every time I mess with the shimming. I've ordered another scope rail, so I can pin this to the wooden block for consistent alignment and then tighten it down with the screws.
I've since pinned the existing rail and ordered some 0.015" steel shim and another rail (oh the joys of E-bay)

All in all very promising, but it shows that I probably need to use slightly larger screws in some positions and possibly be using Loctite to secure them. I also need to regularly check everything for tightness.
Interestingly my brother recounted how his good quality gas power air rifle had drifted out of adjustment after a period of storage ... 1/2" out at 20 yards (I'd be happy with that on the crossbow!)