After some advice please:
The heater core started dripping coolant, can't have that so removed the 26 bolts surrounding the top of the A/C box in the engine bay, and with much difficulty removed the top of the box, then the heater core. Job half done! Cleaned all the rubbish and old coolant residue in there, and was looking forward to a better heater. Incidentally, the blower motor was quite stiff, drawing heaps of current, but after a bit of lubrication is back to 'normal'.
I have had the core replaced, and am working on putting it all back together. I did a trial replacement of the A/C box, and couldn't get it to fit back in - there is significant interference from the hood/bonnet at the top, and edge of the engine bay - I stopped after 3 attempts. Putting the new parts back in means a bit more finesse than when taking it out, because all the old gaskets/seals need to be delicately kept in place. So I am seriously considering lifting the hood about 30cm to get it back in.
Has anybody done this before?
I have the 1979 Manual - which is pretty specific - but it does not mention removing the hood.