Hi Mike
My engine light is on permanently since I had a gas installation a few months ago. I took it to a couple of garages who plugged it in, couldn't find any problems, reset it and within a day it was back on. I stopped worrying about it.
Mine switches over after 4 minutes first thing in the morning, slightly quicker if I leave it in the heated garage. I had the same worry as you, I work from home so only do a short school run and I worried it wouldn't switch over in time for me to benefit but by the time I've got the kids in the car and locked the house it's running on gas.
For the rest of the day it switches to gas within a minute.
The petrol gauge drops as if it were running on petrol all the time, then suddenly, after a few days it seems to recalculate and realise it hasn't used any petrol and the gauge pops back up to full.
I use a quarter of a tank of petrol every month so should be filling up about 3 times a year. My neighbour who has a piddly little 1.4 litre Golf on LPG, fills up twice a year so I reckon I'm on target there.
It's not the most economical engine for the short runs I do but that doesn't matter because I'm only spending £40 a month on gas - compared to the £80 I was spending on my last car, a 3 litre V6 petrol.
And I paid considerably less for the LS430 than I would have paid for a brand new Panda. So it's just win win win.
Good luck.