Its not a very glamorous subject, but correct repair of punctures is a very important aspect of tyre safety.
Firstly, it is important to be aware that damaged tyres just cannot be repaired; sidewall cuts and rips, bulges in sidewalls and out of round separating tread are all death sentences for your tyre. They must be discarded and replaced with a new tyre or safe second hand tyre.
Other considerations in considering a tyre for puncture repair are that it has to be of WOF standard, not been run flat and have no internal or external damage other than the puncture.
Often consumers ask if an inner tube can be fitted to a tyre that can not be repaired. Well, there is no such thing as tubes for tyre sizes 17″ and larger and using tubes in this way is not an approved mode of repair. Tubes can not restore strength to damaged tyre sidewalls and do not prevent entry of water into the tyres steel belts which in due course can lead to blow outs. Also, tubes are not suitable for many alloy rims and tyre profiles lower than 70.
You should always refuse to have your tyres repaired with the illegal string repair system. The big problem with illegal string puncture repairs is that they do not stop air leaking out and let water to seep into the area surrounding the steel belts and allowing them to rust with the consequence the tyre is liable to blow out.
Illegal string repairs are grounds for instant WOF failure. Reputable tyre shops never repair road going tyres with the illegal string repair system.