In reply to steve-grigg:
Depends on how many nights.
If you are one night or two and don't mind smelling, then camp for cheap and within walking distance of the pub in Bosherton.
If you need a shower then class it up in St Petrox at marginally higher tax but drive from the pub. That's basically the choice. Good side burns on the old farmer at Petrox.
Have seen other students camp in the layby between St Petrox and Pembroke for weeks on end.