I think Dr Craig a police surgeon with 30 years experience in 1985 would recognise opium poppies since he would be familiar with morphine.
We don't how many poppies were growing only that Dr Craig and Chief Sup Harris looked at a small area in a field. This could mean they simply looked at a small area in a field although the poppies were spread over a much wider area of the field. Or it could mean the poppies were confined to a small area of a field.
It appears harvesting and processing is not as complex as originally thought:
"Crude laboratories in the field are capable of refining opium into morphine base by a simple acid-base extraction. A sticky, brown paste, morphine base is pressed into bricks and sun-dried, and can either be smoked, prepared into other forms or processed into heroin."
https://en.wikipedia.org/wiki/Opium#Harvesting_and_processing
How does being a doctor make him a plant expert?
First you have to harvest the opium gum from the pods which is a pain in the butt. Then you have to process it into morphine base then into Heroin base and finally prep it for use.
Transforming morphine bricks to heroin base requires a controlled substance- acetic anhydride,. It in controlled precisely because it is used in heroin production. How pure it will be depends on how you cook it and what else you use. Farmers don't bother going through the time and expense they just sell Opium gum or morphine bricks.
There is nothing to suggest that Jeremy or anyone else was buying chemicals and experimenting with opium poppy in order to make their own heroin.
How big was the field? Cultivating 2.5 acres of poppies year round will require 3 pounds of seeds. You can get 3 pods a plant maximum during a single year. There will be 60-125,000 pods each cycle X 3 cycles=275,000 pods max. This will yield as much as 28 2/3 pounds of opium if you do everything right by taking care of the crop and losing nothing while harvesting the opium gum. This can be processed into a 3lb brick of Morphine which in turn can be processed into maybe 2 pounds of Heroin.
Obviously a field cultivated only for months would not produce as much. If you do only 1 growing cycle instead of 3 you get 1/3 as much. Who the hell wants to hand harvest 60,000-125,000 pods? You think Jeremy was going to do it himself? There is a limited time to harvest it all so you need a number of people doing it.
Aside from it being too labor intensive for Jeremy to bother with, planting it along a main road instead of an interior hidden field would be extraordinarily stupid.
