I have found loads online about arduinos and relays, but only small voltages
What i am aiming to to do is have an arduino connected up to a sensor (light or some other imput i choose) and when (for example) it gets dark out side switch one plug socket off, and another on
When it gets light, vice versa
These sockets are standard UK 3pin sockets. I am assuming that a shield doesnt exist for this voltage?? If not, can someone point me at a relay that will work, and can be switched by simply setting a pin to HIGH or LOW
Look into solid state relays (SSR) Solid State Relay - 8A - COM-10636 - SparkFun Electronics. Although, you mention a 3 pin socket, which leads me to assume you are looking for a relay that a device can plug directly into. With the SSRs, you would need to cut up your plug and attach the wires directly to their corresponding terminals on the relay.
What would probably work the best is to have teh Arduino connected to an SSR (solid state relay) and that SSR in turn switching a standard mechanical relay. Typpically you can get Normally Open and Normally Closed contacts on a relay so this would let you switch off one socket as you switch on the other. Either the daytime socket or the night time socket would be powered, but not both.