Sounds like a tricky one mangos. If the houses are tiled/slated as one, the chanches of it coming from the roof are small. If there is a raised capping between you and the neighbour, it may be that you've got a problem there with either the flashing or water ingress through the capping. Go into the attic and check. If everything is OK there, it may be condensation coming from very dense render (waterproofed). If you put on the heating very high and in bursts, the strong render would be cool and therefore the warm air would cool quickly against it. Do you keep the kitchen door open during sustained cooking?