wet patch on bedroom ceiling along party wall approx 1m from outside wall.source unknown.no tiles cracked.no pipes or tank in ceiling

Hi You need to get up there and have a look as it could be running down the wall and entering in loft space some where.

a leak on a roof does not always mean the water is getting in where the wet patch appears, the water can run along a joist or purlin, i would ask a roofer to take a look

You need a roofer to get up and take a look, as you say gutters havent been checked, check these. You mention a cement pitched valley on your neighbours side that is cracked, where does this run to in relation to your leak. The water could be entering there and finding the nearest exit to leak, thats why you need some one to take a look.

Maybe the gutter may be over flowing due to blockage causing damp, if his valley were leaking, it is possible for the water to travel down the felt and into you roof space of course, impossible to say without inspection best of luck