Excess fuel consuption, lack of power

Have a 1.4 seat ibiza that ran great until it overheated during an mot test. Since then it lacks power and fuel economy has dropped to under half, 28 km per gallon. There is no obvious sign of head gasket failure, ie oil in water, smoking, steam water loss, or high oil pressure engine overheats if air con is switched on. Original problem was at the mot (ITV) here in Spain, they could not get correct readings for the emissions test so they revved the engine without checking the temp, and unknoown to me the thermal switch had failed so the fan did not cut in so engine overheated and blew heater matrix, when they tried to start it straight after, it sounded like the battery was flat the engine had sort of seized, although after letting it cool and replacing the water it started ok. But with no power and poor fuel economy. It also sounds like the exhaust is blowing but cannot see any obvious leaks, the engine sounds more like a diesel than a petrol engine!Any one can help it would be appreciated