Insulating external walls of 1930s cottage

Rubble stone walls need to be able to breathe so if you wanted to do this properly you would be looking at removing the existing render as it's likely to be sand and cement and replace it with lime render externally and insulating lime render or hemp or similar breathable material internally. On top of that you should install perimeter drainage to help prevent water runoff from being soaked up at the base of the walls. New rainwater goods would be essential too. That's probably the best thing to do but it is expensive.

One thing you could omit from the above is the removal of the external sand and cement render. You could accept this, make sure it's in good shape but you're relying then on the walls drying to the inside. Which they should do eventually.

Looking at doing this for a project myself at the moment and I'll probably go with the latter solution for now. It still gives me the option to remove the external render in the future should I still experience problems.

I've seen people use EWI in similar situations. I'm not sure about it. You have a lot of wall to heat before it hits the insulation. EWI doesn't rectify dampness in the wall either. In fact, it will probably make things worse in that regard