I need to camp overnight near Liverpool (approaching from the South-East) on Thursday 17th April (First night of Easter weekend). It must be close enough so that there's no chance of missing our 10:30AM boat to the Isle of Man on Friday, but rural enough that we can get a good night's sleep.

Can anyone recommend a site?

marriot hotel in the centre of liverpool is only 45 quid for a twin room during the week...so long as the mighty reds aren't playing in some obscure european tournament at home that night. 10 mins from the docks. i don't like slumming it!!! feel free to disregard this message but don't blame me at 3 in the morning when you're awakened by snorers and you could of been just returning from the gym/sauna/pool/pub/nighclub/pizza shop/mcdonalds/house of ill repute.....take your pick

Make sure you weld your bike onto the car
though, or better still take it to your room and
handcuff yourself to it overnight, and put
clamps on all four wheels of the car and
remove the distributor cap... scallies!

When I travel over to the UK for events on the evening ferries (i.e. the reverse of what you are doing) I use one of the sites close to Chester. There are loads of them in the area. Then you can travel up the M53 Wirral to Birkenhead to the Mersey Tunnel (toll �1) which conveniently spits you out about a mile from the pier head at Albert Dock.
Ring Chester Tourist info for details of the sites.