Hi Tony,
I've been looking into this for the last couple of weeks, having decided to find out a bit more about the whole LPG story.
I can recommend a company called Alternative Automotive Fuel Conversions (01908 641333) on the strength of the information they sent me and the conversations I've had with them; of particular note is the fact that the chap there has a 740 and has already done the conversion to it; with no problems.
He was at pains to point out that if you're going to do it, make sure you go for the fully sequential "Prinz" system (Dutch manufacturer), and not the cheaper, single point systems (often Italian); these are both more wasteful, less reliable and less appropriate to a V8 block.
Problem I have with the whole LPG idea is that a) 7 Series are not on the "Powershift" register anyway, therefore we can't apply for a government grant for the conversion, and b) cars over 5 years old can't apply either. Seems ridiculous to me.....
Other thing to bear in mind is that you really need to be doing serious mileage before it starts to pay for itself; e.g at average 24 mpg and doing approx 12k per year, it would take approx 3 years to pay back; the estimated cost of fitting would be approx £2,760 for your car.
Of course if you were scooting about town and only doing 18 mpg, then the payback would be just over 2 years.
Cheers
Andy