Ford Repair: SPARK PLUGS, ford fiesta, heli coil

Question
I changed 3 out of 4 spark plugs on my 1993 Ford Fiesta 1.1. The last one started to undo but then got all tight again. I approached a garage about this problem and they said it could cost me £££ if it goes wrong. Is there an easier way, can I sort this out myself without paying so much money.  

Answer
If you mean that you were tightneneing it and the threads broke loose you may have stripped the threads where the plug goes in the head.  It could cost you a lot of money.  If you know how to install a heli-coil you should try that.  If the engine seems alright, and the plug is tight, dont touch it, dont ever change it either.  Better off leaving it in if it seems tight enough.  Once the threads have been striped, there never the same and removing and installing a plug in that hole will only make the problem worse.  Worst case scenairos will be that the head needs to be threaded for a larger plug or will have to be replaced entirely.