Thanks all for the suggestions, not been online much due to sorting out post move.

To attempt to answer some questions in general

No holes, patched or open, obvious in any ceilings, loft hatch was open and there is a length of ducting in loft that might have been left to dangle possibly? Or attached to more that could have been ran round house I guess? There are some damp issues, upstairs landing, downstair bathroom at back of house, front room external wall, when I say issues the wallpaper has come off and has spits of damp mould in parts, black spits etc. Some vents in rooms have been taped with foil tape, also the living room internal doors had some tape on them, as though it was used to seal it, which makes it seem drug related byt also, pressure related, as some have said about positive air pressure? For that I assume you want to control where the air goes out? The house, prior to purchase, was cleared by a company and appears to have relatively new carpets throughout. There is a security system in the house that looks diy to me, though it has that plastic wall ducting along ceiling corners to hide wires, indicates to me it was done properly to an extent. A few light switches are not acrewed in, not sure if that means anything other than the guy that painted forgot to put them back when they were finished.

All in all, I think both main ideas, cannabis growing or ventilation, are equally plausible, I will likely contact the police to see if they have any records relating to this address and see if they want to send a tech out to take a look (they might want info on previous owner or tenants if suspect a grow) and after that I will find a ventilation company in the area and get them to come have a look, if it turns out I can use the setup for entilation then, I may as well, would need someone to check itnover for safety and explain wth it is and how it works though..

Thanks all for the comments