The ADFS stand for Air Fighting Development Squadron. They were used to develop tactics for aircraft that were joining the RAF and develop tactics against the latest enemy aircraft. This included flying enemy aircraft in simulated combat against current RAF aircraft.
The paper didn't go into detail as to what the problems were with the P61 given to them for testing but they must have been significant.
Not sure if this is the same report, but here's some notes I made a number of years back at the PRO, when I was Important and Travelled The World.
"AVIA 18/1110
Original reference: 2nd part of A&AEE/821
Test of P-61 at Boscombe Down 6.5.44 - 6.9.44
20 October 1944
"Interesting engineering features"
The spoilers are light and reasonably effective at all speeds from the limiting speed of 380 mph down to the stall. At speeds up to approx. 200 mph the first 30' (approx) of wheel movement from neutral in either direction with very little response. Control is therefore not good in rough air at crusing conditions due to coarseness of the movements necessary. It was felt the spoilers did not produce more response than a normal aileron, with less "feel".
3rd part of A&AEE/821 Brief Handling Trials: P61-A
Combined commentary of a number of civil and military pilots who tested the aircraft.
Bad: Cockpit layout, view, lack of harmonisation of controls, excessive changes of trim over-sensitive trimmers.
Good: Spoilers effective even when aircraft stalled, but large control movements were required, especially at slow speed. The stalling and single-engine flying qualities are notably good but the aircraft is not considered a suitable type for night-fighter duties.
Sent to the A&AEE primarily for armament tests.
Layout: Complicated and confusing. Seat too far from controls, uncomfortable due to being too far from the controls. No fore aft adjustment of the seat is possible.
View: Fairly good
Spoilers: Gearing too low - need 160'-180' movement to get full displacement
Trimmers: Too close together
Exit: Pilot emergency exit is upward, but there are no hand-holds!
Controls: Lack of harmonisation in level flight. Elevator extremely heavy, rudders moderate, lateral control light
Rudder: At high speeds, "snaking" occurred if the rudder was displaced
Trim: One of the worst features of this aircraft was the excessive change of trim with speed, power, or position of the aircraft, flaps oil cooler flaps
Good on one engine
Stall at 80-82 mph."