Well I did it, (it works)
Soldering was quite easy. The gold contacts I originally scuffed up for soldering the battery lifted soon as I applied heat showing a new layer to solder onto the battery terminals underneath. (like another terminal) Because these had been primed / fluxed when making the battery simply touch soldering the wiring harness on was a breeze in just one second for each terminal which helps a lot keeping the battery cool. It fired straight up and is working no problem.
(I have since cleaned up the wiring and moved it forward, neater than this)
Removing the harness from the old battery was also easy, You carefully cut the battery tab open (no electronics just a piece of paper basically) and unsolder them from the circuit. So in theory doing this carefully the old battery could still be used again if ever needed. You don't need to destroy it.
And one working NWZ-Z1050 with a new battery.
There is however one small problem, due to the battery being a little thicker the back case will not fit on perfectly., it is about 2mm up still. This however does not concern me as I can sand down down the ridges with a dremel tool later and apply a thin layer of electrical tape along the side. (a small price to pay to save this device)
I still need to test charging and run times, so I'll report back. (but this can be done)