Another weekly update (do let me know if this is boring and I'll do less of them )
I've now mostly fixed the cocking linkage to the point I can sit and shoot the thing as much as I want without anything breaking. Probably had about 100 shots this weekend.
So now I can start testing in earnest! First job is to get the power up to something reasonable.

The stroke has been upped to 63mm (38.8cc volume). Unfortunately I only seem to own quite soft springs (i.e TX200 mk3) which isn't ideal for this bore/stroke. Muzzle energy has stalled at about 4.75 ft-lb @ 20% efficiency.

Next step is to have a final tweak to get to 65mm stroke and fit a higher rate spring (maybe something like a Titan #7 or an OEM equivalent from something). Then start slowly increasing TP diameter to see where the sweet spot is.
Hoping to get up to around 10 ft-lb, then I'll start balancing the masses and see how much recoil can be removed.