rover 214 sei 16v headgasket blown :(?

I have a rover 214sei it really like this car and i am reluctant to let it go the headgasket blew so a mechanic friend put a sealing agent into the engine and it didnt work or so we think, the problem is as follow's the car reaches it's normal running temperature then after about 5 mins just dies i can keep it going if i rev the B*LL**KS of it but as soon as i press the clutch it just dies and on come the lights then i cant get it to restart after this i have to wait ages about 20 mins then it just's get's me back to my house not very good any Help would be greatly apprieciated.

Roger Williams
June 2005