Just a quick update on this project...!
I eventually tried the caustic soda method by pouring the solution in through hold in the bottom bracket. It fizzed, got hot etc and I left it for a few days rinsed and repeated a few times. As there appeared to be a lot of solid seat post in the frame I'd thought I'd give it some assistance with a drill...
A few broken drill bits later I decided to pass it on to my old man who works in metal fabrication. He assured me he'd have the hardware to drill through anything...
He fixed the frame up to a machine, but he could only drill about 2-3 inches down into the post until he hit something else that was too hard to drill through. Could this be some kind of residue left behind from the caustic soda?
As expected, loads of black liquid stuff came out when I rinsed the frame through after using the caustic soda. Just wondered if anyone else had seen something like this before?
Thanks!