I've had a read through of that page, there's nothing in there that suggests that it was the intention of Section 5(1)(ab) to exclude air weapons.
Unless I've missed it?
Ah hang on...
paintball guns are a type of air weapon. The Home Office regard self-loading or pump action rifled airguns (including paintball guns) as outside the scope of the Firearms Act, unless they are sufficiently powerful to fall within the category of a "specially dangerous" air weapon (Archbold 24.8a). Paintball guns could be considered imitation firearms.
That's a pretty useful quote.. it's a bit vague unfortunately, but it does refer to self-loading rifled airguns as being outside the scope of the Firearms Act. It's a shame it doesn't specifically say Section 5 of the act, but I think that's what it's implying.
That might stand up in court I reckon.. It would be a pretty good defense. The problem you'd have in court though is that Section 5(1)(ac) does specifically have an exclusion for Air Weapons.. which sort of contradicts the CPS quote - It's difficult to argue that the intention of the act wasn't to include air weapons when one of the paragraphs specifically mentions them.
Anyone have anything else?