I have just purchased a Leatt Comp 3 Neck Brace. I did remember reading somewhere on the web that the helmet I own, an older Spesh Deviant is not as good with this brace as other helmets. I’ve tried them together and the moveability seems fine to me; I just don’t want the safety and effectiveness to be compromised at all by having a helmet that doesn’t do the job as well as others.
Is there anyone who can give me some advice please.

I suggest selling it to avoid spinal compression injuries, but that’s just me.

Try making adjustments on the neck brace. I’m assuming it “won’t work” as the shape of the helmet limits movement too much.

For what it’s worth, I’ve got a Bell Drop and a DBX Ride 3 and I’m quite happy with it. Good movement when set up properly. I find the chest strap useful too.