OK well I have progressed onto the next step of re-installation.
I used a zip tie to hold the special tool in place after a couple of slips. Slotted the tensioner straight back in, cut the tie and the tool popped off and the bolt holding the tension fell out. All good.
I got a set of safety collars for the tensioners
After a bit of fiddling, I get them started and.....oh
There isn't enough tensioner shaft showing to attach the collar + the spacer tool that comes with the collar, and still have any shaft showing at the top.
I checked the photos I posted before I took the tensioners off - it's the same amount of tensioner showing...which means that that's just how it is with the chain on.
I did try turning the motor a few revolutions and prying the idler arm upwards to take out any slack in the chain...only a minute change.
I guess I just have short chains?
Putting this question out to those with learned opinions:
(a) ditch the collars and go without them
(b) do them up at a lower distance off the top of the tensioner (i.e., use a thinner spacer than the supplied one
(c) use the spacer even if it means the top of the collar is flush with the very top of the tensioner (which is slightly bevelled, meaning they might not have 'grip' all the way along)