You can't really leave your car in the St. Govans car park overnight they take your Reg number and leave you a slip detailing local byelaws which prohibit overnight parking / camping. (
).
They even do this when its bucketing down and so sane person would be wandering around taking number plates.
There's a layby just outside St.Petrox that you can park up in and usually has a few vans.
Ma Westons has a loo and so too does the pub. If you're feeling less pikey then I'd suggest eating at the Stackpole Inn. Fantastic food which shoots the St. Govans down in flames.
BTW - I wouldn't be trusting the weather forecast, its piss wet through here at the minute and was too yesterday. Sun isn't getting time to burn off the early morning grease before more rain hits.