Roofing issues - advice needed

Physiobro

I will try to be as concise as possible. See photos for context  
Jan 2023 - re-roof with existing stone slates, fitting of felt, fitting of velux windows, cost of £6000 
Jan - April 2023 -  always been an attic room in this house, no insulation so I knocked the lath and lime ceiling down. Then insulated the room myself leaving 40mm gap between the insulation and the felt for air flow. Overbattened the existing rafters to deepen them, put 60mm solid PIR insulation between them and 40mm over the top of all of it in a continuous layer, was meticulous about gaps and used tape and expanding foam to ensure air tightness. 

April/may 2023 - room was plasterboarded and plastered then I decorated 
The plasterboard meets the lime plastered chimney breast wall as shown (patched and filled by me with one coat lime and toupret interior filler)
June 2023- noticed water ingress on chimney breast wall and along the perlin (beam). During rainy periods the water physically dripped along the perlin and on to the floor. 
Had a 15 year guarantee from the contractor covering their work so original roofing contractor took a look, blamed a loose slate, apologised. 
July to October - issue persists, contractor hard to pin down, came again eventually and blamed issues with the chimney stack which wasn't on original quote. Personal relationship had broken down by this point and he refused to do any more work. 
Had no choice but to get someone else in who repointed parts of the chimney, flaunching and ridge. 
Issue still persists and is only getting worse, had lots of different opinions but none that really fill me with confidence. 
I've accepted that the chimney pots need capped and am looking in to getting this done. 

Main questions: 
Why is the chimney breast so wet when this issue wasn't there before having any work on the roof? 

Could the water on the perlin be a condensation issue with water running down the top side of  the new PIR insulation to the point where there is no insulation (on top of the perlin) and coming through the plasterboard. Why does this only happen when it rains though? 

What do I do next? I'm at my wits end. 

grumbler

The chimney - I'm no expert, but I think  the led flashing has to go all the way up to the ridge and over it. IMO, mortar where the ridge meets the chimney is unacceptable.

Doozergirl

I'm not a roofer. By the look of it, the person that carried out that repair isn't either.  It's really shoddy.

The purlin issue is likely to be water running across it and/or the gap running between it and the PIR. 

From the marks inside, I immediately thought chimney, before seeing outside. 

Get a qualified roofer to look at it.