It was the same installer I used both times.
The difficulty apears to be the engine suitability with some engines you are just asking for trouble.
What car are you thinking of buying?
Is it a multipoint system? as these systems offer significantly better performance and ecconomy (you lose about 10% in performance and fuel ecconomy with LPG)
PS Make sure it has the instalation certificate as you wont get insurance (and cheaper road tax) without it.