Thanks Psyche. The teeth on this model are a bronze gear that I will remove before welding. It's the fixing for this gear that I think corroded and created pressure that lead to the, as you note, quite a crack. I have loosen that fitting (bottom left hand in the photo below). I was going to ask the engineer to drill the hole rather than attempt it myself, but is time of essence and I should do that today?
Thanks for responding
Kieran