Thanks for the input all, very much appreciated.
Had another roofer take a look and he's advising two things...
1, Re-point the chimney on the roof side and remove/re-bed the pot - showed me the pictures and this definitely needs doing, poor condition and cracks - quoted £250
2, The felt runs beneath rather than lapping into the back gutter so if there's water getting through the tiles it'll run into the loft rather than into the gutter - again, showed me pictures - quoted £200 to remove felt/batten in vicinity, re-felt and re-batten ensuring laps into gutter
I should be able to check if water is getting to the felt from inside the loft so at the moment not convinced I'll go ahead with 2 as while appreciate not optimal might not be the problem.
I've asked him to send me the pic of the gutter he took and will share on here if he does for opinions and hopefully settle some of the discussion hah!
I'm waiting on another opinion, but difficult to get roofers at the moment round here so may not show.