I have a Vauxhall 1999 5 door 1.2 16V Corsa, blows cold air, temperature gauge on red, whats the problem?

A garage has told me it is either the thermostat, water pump or heater unit, if none of these it could be the head gasket....? I have taken the oil cap off on the engine and their is a light brown 'gunk' on the cap...does this mean the head gasket has blown? is there anyway I can test or find out what the... show more A garage has told me it is either the thermostat, water pump or heater unit, if none of these it could be the head gasket....? I have taken the oil cap off on the engine and their is a light brown 'gunk' on the cap...does this mean the head gasket has blown? is there anyway I can test or find out what the exact problem is without spending a fortune?
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