Pure speculation on my part, tin hat firmly in-situ, but I'm inclined to see EP having something on some of the people that testified against JB. I believe telephones/properties were being listened in on: Bourtree Cottage, Morsehead Mansions and JM's shared house in Caterham Road.
Those that testified against JB: JM, LR, SB, James Richard (JR) and Malcolm Waters (MW) were all connected through JM.
LondonJM house shared with SB and JR. JB took packets of cannabis to the house for JM to distribute on campus. Were SB and JR also involved in distributing cannabis?
EssexJM worked with LR and MW. MW was the owner of Sloppy Joes and LR was the manageress.
LR had relationships with MW, Charles Marsden and a one night stand with JB.
MM supplied JB with hemp seeds for growing cannabis plants.
JB also worked at Sloppy Joes.
All the above hang out at the Frog and Bean wine bar owned by MW.
JM admits to smoking cannabis. Suzette Ford also smoked it. MM too. Was JB growing it and selling it via the Frog and Bean and/or Sloppy Joes?
JB grew cannabis at Bourtree Cottage. Prior to this he grew it at WHF until NB discovered and put a stop to it.
It's possible the police said they had enough evidence on JB to secure a conviction but wanted them to testify and if they didn't they would be charged with drug offences:
https://www.gov.uk/penalties-drug-possession-dealing