Flowers from heaven, there are a couple of things that you can do.
One is to do an internet search for the property at the time that it was up for rent - naturally, this would be a few months before they actually moved in. That way, you can find out the agent and the terms.
Two - the Land Registry information usually states who the mortgage provider is. Report it to them, as it may be possible that the property was not purchased as a buy-to-rent.
Three - get a large block of wood or timber and some ear plugs/defenders. As soon as you can smell the cannabis through your chimney, start drilling. If anyone asks, you are making a sculpture from the piece of timber with your drill. Hopefully, your neighbours will associate the awful noise with their puffs.
Four - as soon as they start smoking in the garden, light your barbecue and start to 'cook' weeds such as dandelion and nettle. If anyone asks, you've seen this done on the internet and are giving it a go - you've heard that garden growth is good for the blood.
If you don't mind 'face-to-face interaction', you could approach them to ask for the landlord's details, as there seems to be water coming down the chimney at the common party wall on your side, and you will need surveyors to access their property to inspect, as the chimney stack might collapse onto their property if action is not taken soon.
Good luck.