= Should I repoint and replace this with lime plaster on a external solid brick wall =

Do you have damp issues here or is this just an exploratory?
I live in an old stone and brick cottage which has been rendered with cement inside and outside, and it would have been too much work to strip and rerender with lime, but it depends on personal circumstances. You will find that trying to remove cement mortar on old bricks is when the bricks crack so watch out for that

Victorian brick homes are usually a higher quality build than my type of house and I wouldn't expect an interior wall to need to release vapour. Would be better to let that happen on the exterior. I'd check gutters etc on the exterior to see if there is any obvious signs of water ingress

Anyway, good luck!
Thanks for your response!
This is just an exploratorybut we had high damp readings on our buildings survey (not sure how reliable that is to be honest)

In any case, we would like to insulate it in the inside as rendering on the outside is not an option (we live in a conservation area), so we thought it would be sensible to lime plaster the wood fibre boards onto the brick. That's how we got to this point

This is an external facing wall of a Victorian house. It is made of solid brick. I am not sure but the colour of the plaster appears to be cement, and I read that it doesn't go well with bricks as they need to breath. Also, the bottom half of the wall has glazed bricks on the outside, and the top half "standard bricks"

Should I take off this plaster and add lime plaster instead to prevent the cement from breaking the bricks and allowing moisture to pass through?
On top of that, may I insulate with a layer of lime plaster > wood fibre boards > lime plaster > breathable paint?
Are you sure it's cement based?
It looks like a cement colour, but dried cement mortar/screed would usually be lighter unless it's a very strong mix (and even then would tend to be a bit lighter than your photo, and you wouldn't tend to be using very strong mixes to screed a wall with, and if you did I suspect it wouldn't have come off as easily as this appears to in your photo without damaging bricks)

The texture of it, and that the same stuff appears to be the mortar between the bricks might suggest that it is lime and has just had additives that have made it darker?
It's not uncommon for lime to be mixed with ash, charcoal, brick dust and all sorts of other stuff to influence the chemical reaction (like making it set faster for example)

For example:
Before you think of hacking it off I'd try to be sure that it is actually cement based as you can't just go off the colour alone (though that is usually the most common difference)

P.s the bricks being different at the base of the wall outside may be because they've used engineering bricks there (this is a good thing)

I am not sure this is cement! Is there a way to confirm it?
I uploaded a couple more pictures with a closer look, and it looks pretty similar to the link you posted. You can even notice it has a few "hairy" bits in its texture. Not sure what that is though (sorry it is not imgur, it seems offline atm:
Thanks for the advice, I will have a look there. At this point we are trying to gather as much information as possible before weto something

Do you happen to know if original plaster and mortar should need replacing, like this one? This seems pretty solid on the wall but easily crumbles if I try to drill, making alarger than I wanted if I am not super careful. I suppose mortar is supposed to give up after I while but I am wondering if that's enough reason to get the whole house re-plastered after time.