Ford Ka - I forgot to put water in!?

Kwik Fit do not do head gaskets as its a job for a professional mechanic not a part fitter. It will normally have to go if for a few days as the whole head has to be taken off and maybe skimmed. Best bet is to call out the aa or rac if you have cover as they will be able to tell if its the head gasket or not. There are many reasons why it has overheated and although you forgot to top up the coolant theres no reason why it should be empty. It could be as simple as a thermostat or radiator leak. If it is the head gasket it could cost in the region of �200 to �600 to repair. Good Luck