bp wrote:

The whole wall externally is painted black (all the way to the chimney
stack). I originally thought this was just exterior paint
(weatherproofing), however one builder told me that it looks like
bitumen paint. He says it may have been put there to prevent damp
getting in, but in reality perhaps it should not be there as by default
it also prevents the wall from breathing and could be the cause of damp
problems internally. So here are my questions:

How can I tell its bitumen paint

Wipe it with kitchen roll soaked in white spirit. Bitumen is brown in
solution although it looks black in a solid form. I can't imagine anyone
would choose black to paint a wall.
Whatever it is, it sounds like it should come off and the procedure
would be the same. If it's on the bricks, paint stripper (probably 25
litres plus). If the wall is rendered, might be easier to chop the
render off.
I don't envy you. It's going to be an expensive job and I think I might
look for obvious places where the coating has failed before committing
myself to drastic measures. You might be able to patch those areas with
something similar.
There are 2 basic approaches to this. One that the coating is
impermeable enough to prevent moist air getting out. The other (and much
more likely IME) that it's leaking and allowing water in. Observation in
different weather conditions might give you a clue.