Coreplug issue solved...

Well, thanks to my friends at MG Experience I've found out all I need to know about MGB core plugs.

Turns out that there is one at the back of the engine block, but it sits behind the gearbox adaptor plate. There's a hole in the adaptor plate but it's not big enough for the coreplug to pass through. There are 4 core plugs in total - three along the non-manifold side of the block and one at the back of the block and all are the same size. To replace the rear core plug you have to split engine and 'box.

It's good to settle that little question and work will now continue to remove the engine from the car...