right got an RS Turbo converted cab,as far as i can see standard apart from stainless exhaust system,dirty dump valve,and incar boost controller

anyways,they guy who i had it off had by passed the heater matrix,due to when the converison took place one of the pies tot he heater matrix had been damaged,so i got it home drove fine all the way home with out fault no over heating,(drove it over 14 miles home)took it out the next evening,got stuck in a little traffic and i could see the needle rising up,so was getting very worried by this point,drove it straight home and switched it off,
thought there might have been an air block in the cooling system so ran it up to see if there wat any air trapped,it was just above running temp and the header tank started bubbling up badly and started to over heat badly too,the fan isnt cutting in either

theres no oil/water mix is it the headgasket?failed water pump?cracked cylinderhead?wot should i look for?

is it a big job to do myself?exspensive?

any advice appreciated

cheers Jez