Penetrating damp

Lived here for 13 years - house is 23 y old. Extension done about 19 years ago .. by cowboys I suspect because I keep finding blunders! (e.g. roof had to be reinforced with steel when surveyor noticed that timbers were undersized! .. tried to trace them on another matter but the firm has ‘vanished’).
So it’s damp patches - one up, one down adjacent to external west facing windows (roughly 30cm by 30cm) . .. not directly above one another. Double glazing fitted about 10 years ago but I “think” that these patches had always been there – not severe but some efflorescence, spoil of decoration but I suspect plastering will be needed .. want to decorate these rooms at some time .. don’t want new plaster/decoration to be spoilt so how to fix damp first?
Cavity wall insulation fitted also about 10 years ago .. that could be the culprit .. or bad wall ties from original build but as the cavity is now filled with rockwool (or something similar) can I really ever hope to find those bad ties?
I’ve done some research. Products like ‘Stormdry’ seem to last a long long time (20-30 years according to the data sheet) and at £125/5l would probably be cheaper (?) than having a builder to work inside the walls; plus bonus of masonry would last longer and thermal efficiency would be enhanced.
The wall is 2 storey+loft and is totally exposed to the west and at the top of a rise so every rainstorm seems to batter it. UPVC fascias.

Would a high quality sealant stop the damp penetrating?

Any advice from the experts out there please?

Check the window and the sealing around especially at the sill. And anywhere else you can see water on the outside or debris against the wall. If you've a blocked cavity it's got to be coming in through somewhere- it may even be the gutter, downpipe joint or roof.
Failing mechanically stopping it, try Thompsons water seal 2 coats hand sprayed on externally
This is an extension - does the main roof discharge on to it. That rain water could spread under the lower tiles/slates or overload the lower gutter. Try a lead spreader plate under the shoe or even extending the pipe away from or over the roof[/u]
The key is to stop it externally, if impossible, then go for the internal render - 3 coats 1st 1-1 sand cement thin layer 2nd 3-1 3rd 6-1 All with waterproofer solution in the mix

Additional
missed that about a 2 story loft
Examine the outside wall above the damp patch for missing pointing etc and and other area of possible damp ingress even up behind the facias & soffits roof edge and gutter

It could be interstitial condensation. It will show up as a damp problem but with no obvious cause.

Goat/Joe

Thanks for this. I forgot to say that it is an end wall so no gutters above or even near.
Had the d/g firm back initially (because I thought it might be the sill seal .. but they said the seal was good; I tend to trust them -- they did a pretty good job and there's been no problems with any of the other windows/doors.
Re the external sealant .. would I have to do the entire wall? I am wondering whether, if not, water could leak in "behind" the sealed part from above?
The reason that I think it's most likely penetration 'near' to the problem is that there is no sign of any problem above the damp patches (which have many metres of clear wall above them
Thanks again form your help.