Turns out it was from the drive shaft as I found another one on the floor over the other side today when I revisted the car! Managed to swap the bellhousing over today and going to fill her up with oil tomorrow to let the sealant cure overnight.
Wondering how to tackle mounting everything back to the car... mount gearbox and then try lift the engine... or mount them both together and try and lift/support the whole lot and frantically try and bolt things in underneath!
Anyway some pictures documenting a little now follow

.
Old con rod shell pic:

Cylinder 1 Piston (304g):

Cylinder 2 Piston (307-308g, I tried filing it down but 4 gram is too much to remove by file! lol.):

New piston pin locking ring in place:

New Timing Chain fitted:

Timing cover after a degrease (now the cleanest part of the engine and the most hidden!):

New piston rings fitted:


(now I hope thats not a crack I can see on both of those! pretty sure its just a scratch but I really hope so!)
New sump gasket:

Assembled Engine on the back garden:

Box dropped off the car and lay down for a while:

Switching the damaged bell housing over for the new one from 1973/126 (Cheers Lad!)

Much better:


(Also replaced the input shaft oil seal inside the shaft/bushing assembly as it came with the gearbox gasket kit it would be a shame not to use it!

.).
So tomorrow hopefully get my hands on some proper gearbox oil for the thing and see if I can mount it all back up for a test run... Before I order the gearbox mounts

. Any tips appreciated! I'm wondering if I can run our engine support bar diagonally across the back seats perhaps and hoist her up but don't fancy it much...
Got to get a couple of spade connectors and attempt to fix the fan as it appears one of the connections there has broken off. Also got to fit the new points and condenser on the distributor tomorrow.