Water was finding its way into the 'weak spot' at the bottom of the valley where it meets the corner of the UPVC window.
I tried taking out a few of the tiles to access the problem area but because the tiles all have 'nibs' & are double nailed, I ended up breaking them (also old & brittle).
When the new windows were fitted, no lead 'up stand' was behind the trim, so this seems to have allowed water to come in & cause a damp patch on the inside wall.
Any how...as Tommy Walsh wasn't available to help with this one .I decided to strip from the top down, hoping that then I wouldn't break too many tiles.
Once the front Plastic trim & tiles were removed, I was able to put some lead in with a 100mm upstand & dress it down around the previously exposed timber.
I think it looks fine & the new lead should stop the damp problem.