Three different types of cannabis - imported marijuana (left), super-strength skunk and cannabis resin
Matthew Atha heads a unique organisation which surveys drug users at music festivals and other events.
Anonymous questionnaires monitor what they are taking and how much they are paying, and the information is used to provide expert evidence for drug trials.
He said his Independent Drug Monitoring Unit has detected a shift in the cannabis market in recent years.
"When we first did our surveys back in the mid-1980s through to the 1990s, cannabis resin was the dominant form in the market," he said.
"In 1994 skunk or home grown cannabis made up maybe 10% of the UK market.
"For the last four or five years skunk has taken up about 70% of the cannabis market in the UK."
Matthew Atha's unit specialises in studying drug consumption patterns
He said a combination of factors led to this change.
"Skunk is generally much more potent than most types of cannabis resin, so people are after a higher quality product," he said.
"Second is the economics, the price of skunk is very much higher than that of cannabis resin ... so the actual profit margins are much greater if you're selling skunk than if you're selling resin."
He claimed the high-strength variety is often the only type of cannabis that users can get hold of, so those who would prefer the less potent resin no longer have that option.
He said more cannabis may now be grown here because profits from smuggling it have "dropped dramatically", while penalties remained unchanged.
The costs of importing are also higher, with payments to drivers and corrupt Customs officials, he said.
Mr Atha said there are various scales of cultivation.
"At the smallest level you may have somebody growing three or four plants in a large wardrobe, which maybe yields about £200 or £300, every three to four months," he said.
The price of cannabis resin has fallen while the cost of skunk has risen
"They'd be used essentially by the grower themselves.
"The next scale up you get somebody using one room in a house which maybe yields round about a kilo, about £3,000 worth over a period of maybe every 10 weeks, so maybe yields about £10,000-15,000 a year."
He said most of this would probably be used by the grower but with a little bit sold on to friends.
"Then you get the sort of small commercial scale where the whole property is essentially turned over to growing cannabis," he said.
"This might yield maybe about five kilos per crop which would be worth round about £15,000 a crop, maybe five crops a year would make £60,000-£70,000 a year.
"On the largest scale you get the industrial scale units or agricultural units which might yield 30 to 50 kilos a crop, maybe worth £120,000 a time or maybe about £300,000 or £400,000 a year."
However, he said his research contradicted police claims that most cultivations are the work of South East Asian gangs.
"We have come across a number of cases which have involved growers of South East Asian origin, but they're a relatively small proportion of our caseload," he said.
"The vast majority of people involved in growing cannabis in Britain are native British for want of a better word.
"You do get organised South East Asian gangs who will bring in people from abroad as illegal immigrants, and make them essentially pay off their passage by acting as a gardener and taking the penalties if the grow is detected by police.
"But, again, at the lower level of the cannabis market it's very much a sort of sole trader entrepreneur, self-employed type of model, whereby people will buy a large quantity at a lower price and then sell instant numerous small quantities at a higher price."
Hash in the Attic will be broadcast on Tuesday, 3 November on BBC One Scotland at 2235 GMT.