Job done! Drive side plug out (see photo)
As RichardL suggested I went with the Dremel to grind the rest of the plug out right down to the threads a slow but controlled method. I did not need to drill all the way through to the crank oilway, which is just as well because the Dremel creates a lot of grit and it would be a worry about getting all of it out. When it came to the levering the plug out it looked like it might drop into the space behind the plug and become a real pain to get out, so I drilled a small hole in the end of the plug (these Allen key plugs are very deep) and managed to wind it out with a small easyout and a long rod helping push through the oilway from the timing end.
Although both plugs show signs of straking in the past to hold them in, this had been drilled out and there was no sign of new straking holding them, the timing side plug did show some evidence of Loctite, but if they had been Loctited why where the too far in?
In the end there was very little sludge in there and I have given the crank a good clean out and blow through.
Back on the timing side (see better photo below showing enlarged hole drilling). My remaining concern is that yes the plug was covering the original oilway but with the enlarged drilling there should have been plenty of room for the oil to flow, so why do my big end bearings look so worn? I need to follow up on the advice you all gave me about checking the rest of the oil system, thanks.
Still does not make sense that someone should drill all the way through the crank web rather than relocate the plug flush.
I have questions about the roller bearing conversion and putting the engine back together so I am going to take some more photos and start another thread on the engine next week. Hope to see you all over there.
This will be my last post on this thread so thanks for all your advice and moral support.
Happy Easter
Degsy