hi looking for some advice.been in house for over 7 years no problems but my roof has now developed a leak at the join with next door.next doors roof is in a sorry state.i ve had a few roofers out and basically the roofs are joined with a mortar joint on a bed of slate.ive been told by most roofer i need a bonding gutter, cheapest price was 330 for just the front,other prices were 800,1300 and 900 for both front and back.spoke to the tenant next door who's also noticed a leak so i phoned the owner, says hes no money to chip in to fix it. so i said i would pay for the work,he said okay but make sure you put the roof back right, but im worried because of the state of the roof next door that i might be opening a can of worms by getting it done and i might get come back from the tight sod next door.ive had another roofer come out today who says best bet,becuase of next doors roof, would be to paint the motar bed and six inches onto his roof with aquaseal.as you can see from the pictures its been painted some stage in the past.im leaning towards just getting the front done with a gutter for 330 and hope i have no come back.any advice would be appreciated, or any other suggestions