See, in particular, Sheet 10 of David Boutflour's police statement of 17th. September 1985.
The lock fitted to the veneer door was a simple nylon ball catch. The catch plate must be out of sight, or it could even be a hole drilled into the jamb. Either way, it is not secure, because you just twist/turn the handle fitted to the veneer.
Before I leave this, here are the possible explanations for P.C. Dryland's statement:
1. P.C. Dryland did not actually inspect the gun cupboard. This is possible because Nevill may have produced the firearms for inspection elsewhere in the house, maybe in the kitchen, and P.C. Dryland may have assumed that, Nevill being a magistrate, and everything seemingly in order, he needn't check further.
2. Nevill previously had a more secure mechanism on the cupboard door, which Nevill himself or somebody else has removed for whatever reason.
3. Jeremy removed the lock on the cupboard on the night of 6th./7th. August 1985, and also maybe left guns lying around the house, so as to give the impression of laxity on Nevill's part in his management of firearms, believing this would assist his story.
Regarding 2, on reflection, a loop/hasp mechanism would not work as the jamb seems to jut out from the wall considerably. It would also not be logical to replace a hasp/throw and catch hole with a nylon ball catch. For that reason alone, I think we can dismiss 2.
Regarding 3, I believe it was Anthony Pargeter's gun that was stored in the downstairs washroom, which suggests Jeremy would have no reason to leave guns other than the rifle lying around; but in any event, leaving guns all over the place and changing the lock on the gun cupboard would just be one more complication to add to his tasks on the night, would leave prints and potentially arouse suspicion.
That leaves us with 1, and that's what I would go for. I suspect P.C. Dryland did not carry out a proper inspection.
P.C. Dryland's statement pre-dates the visit to K.D. Radcliffe's in November 1984, but I have seen no reliable mention of gun cupboards/safes elsewhere in the house and it would seem logical for Nevill to store the guns in the den. If Jeremy wanted regular access to the rifle, Nevill could surely have given him a key on whatever basis.
Whether Nevill misled the police, we can't say. It could easily be that the absence of a padlock mechanism is based on a misunderstanding, with the police officer assuming it was secure and Nevill perhaps not understanding his responsibilities, or not caring, but I think we are being very generous to Nevill there. He was an experienced firearms owner and a magistrate and must be assumed to have known his responsibilities in the necessary detail.
In any event, Nevill does not come out of this well, in my view.