Hey Finally it's sorted !!!

The fault was traced down to the HT block on the front of the top of the engine, this was occassionally not sparking at the correct time and as the 3.0 sparks twice in each of the engine cycle, once as normal on compression and again on exhaust (so I have been told) the petrol was being ignited on the exhaust stroke hence the backfiring, the garage replaced the HT block and the car is great, only one problem still remains and that's the front fog lights, anyone got a wiring diagram or know where I can get one?