Insulation part 1

We want to get the house insulated and energy-sorted.  Im discovering its not easy and straightforward to retrofit an 1860s victorian  terrace.

There are a number of different projects that need tackling.
This is one - sorting out an area at the back of the house - its a cold corner, I  think for several reasons - the solid wall is an external one - the neighbours' room stops short of our probably edwardian extension, there isn't a radiator really adjacent, and there is a very nice Velux window, which lets in a lot of light during daylight hours and is double glazed, but i bet there is some heat loss through it.

There has been damp coming through  - I think from a bush that was growing ito the brickwork, but the wall has been stripped for a while and  seems to be dry now.

Part of the wall has a cupboard along it , so its a relatively small area that is exposed. Ceiling to floor is approx 290 cm and the length of wall is approx 190cm. We thought studs and sheeps wool and then plasterboard is overkill. So its a board of some sort. And I've read that  damp can be a problem, as any moisture  condenses on the cold surface of the wall . So the  joins in the board etc are taped. But what about at the top and the bottom ? Does it mean that it will only really work well if its fully insulated and sealed at the top and the bottom to prevent ingress of water vapour laden air  -   Ive therefore got to remove the skirting board, and the original moulding ( it may eventually all be going - but that doesn't solve *this* problem.). The room isn't used a lot at the moment - so it shouldn't be a problem ( should it ? )

Ive  gone to google ( as you do ) . Heres a forum discussion that includes stuff  about  calculating the dew point of a wall. if I can calculate the dew point, I could show that it may not be a problem and another forum that highlights the problem of excessive condensation and it having nowhere to go

Ive not yet found the 2007  iso standard but there is this powerpoint about condensation (surface and interstitial) . its got 2 pictures -  this of internal insulation (which hasn't got vapour barrier)

and this of a timber-framed wall ( it looks like a stud with insulation between, and plasterboard on top ) - but this has got a vapour barrier .

I think what I need is the Glaser method of calculating the dewpoint.

Some of the insulation boards that you can buy come with  an integral vapour barrier. But will end up not letting  moisture out ?

or are we back to thinking  about studs and lambswool as the insulation , because that will breathe , or what i have seen listed as 10mm flexible liner  - it looks like its 'polystyrene wallpaper'