So it appears that the second nut is a castellated nut, but the bolt seems to have been welded on - neighbour has lent me a grinder, so tomorrow cutting the bolt bit off, and then (hopefully) I’ll be able to knock it through!
Also still to clean off the remains of the old crush gasket and the flange from the central pipe has lost a bit of itself to rust - probably use some gun gum if I need to make up any leaky hole - just can’t face replacing the centre pipe just now - maybe in the summer.
