After reading about subsidence, footing, settlement issues, etc. I came up with the theory that the issue is caused by a number of things:
- there is probably little footing
- the house is built on clay
- the wall is south facing (it gets a lot of sun)
- the gutters empty underground next to the footing of the house in a corner - not sure how or where exactly but I would not be surprised if the ducting is blocked somewhere because it appears to be very wet where the gutter enters the tarmac
- in this area of the country we get a lot of rain
I think what happens here is that the clay under the house expands when it rains and shrink pretty quickly when it is sunny, causing too much movement for the wall to handle. Also on top of that, the water that enters the existing cracks, freezes when it's cold and pushes the cracks forward a bit at the time.
I am not an expert and I didn't get to poke around enough to know for sure but on a second inspection, I realised that that the inside wall is pretty damp (which is not a good sign for a south facing wall - I guess the cracks don't help) and I maybe there are some cracks inside after all - behind the wallpaper.
It is clear that there has been movement fairly recently because the various fixes that the previous owner attempted have popped out - so I think this is a current issue and does need fixing... whatever the issue is, it needs an unknown amount of time and money thrown at it. Perhaps, the best thing to do would be to extend the foundations and the house forward.
In any case, I found other unrelated issues which put me off and the property price was not negotiable, so in the end, we changed our mind about getting this house - it would have been a bite bigger than what we can chew right now.
Thank you everyone that commented on this post... Your help was truly appreciated.
...Good luck to whoever takes it on!