How much do you get paid for being on call?

The company I work for is looking to introduce 24/7 It support for critical systems. I am in the Infrastructure team (2 people). We will be the first point of call for any isuues and they we will escalate if not Infastructure releated.

They are offering a ?200 a week retainer (?20 a night weeknights, ?50 a night weekends) and a ?20 per call or OT, whichever is greater.

Is this reasonable or should it be more?

Your thoughs would be appriciated.