OK – this is the train of logic so far:
Can I park at Llandullas Beach Car Park? The order says, yes, you can, if you are a car or light vzn.
From CQ22
So, what is a car or light van. Helpfully, the Order tells us what the Order regards as a car or light van and where to see the original definition: Article 3 CQ 18
So we then look at the Road Traffic Act of 1984 in Section 136
So … we fit the order's definition of a motor car (and, if needed we can find DVLA confirmation of this. I think Channa has already done so). The Order is specific. We can park there.
There is a possible fly in this particular ointment. The order also provides its own definition of a motor caravan. In CQ19, there is an addition to Article 3:
But …. nowhere in the Order or its Amendments, so far as I can see, are motor caravans excluded. The definition is therefore irrelevant. The order does not exclude them – only defines them. I'm doubtful if, since central government has provided a definition, the council can invent its own vehicle categories. There is already a definition and the Order accepts it. Can the council overrule it? Maybe. I don't know. But it doesn't matter - the order does not exclude motor caravans.
The fly in the ointment is that there is a sign at the car park excluding motor caravans (or motorhomes). This sign is non-standard in that it does not conform to the pattern of signs in the council's other car parks. Why has the council not conformed to its normal pattern of car park signage? It could have been placed there by any interested party – a disgruntled resident perhaps, or a nearby campsite owner wishing to promote his own business. The sign is obscure, inadequate, non-standard and shows no authority.
We are entitled to park there and we are authorised to do so by the order.
EDIT: When I say it shows no authority, I mean that it doesn't look official; it looks unauthorised. I don't mean that it doesn't specify the Order; I don't think it has to. This sign just doesn't look the business - compared to the norm elsewhere it is slovenly, amateurish.