Fuel is now at over £1 a litre average where I live, and if I get this new job I will be doing a LOT of miles. I want to keep the meega, and the only way I can do this now is with LPG. So we've made a start.
Yesterday - all measurements were taken in the boot area, and holes drilled. The tank is now mounted, and fully secured in the boot. As you can see, it doesn't take up that much boot space - this is a 70 litre tank:

Holes drilled in boot floor for tank strap securing bolts:

Where the LPG filler and feed join the tank valve:

I've not yet fitted the filler, because I'm in the process of cleaning it up and spraying it Star Silver, so it's colour coded like everything else on the car. The filler will be mounted in the bumper.
One of more holes still need to be drilled in the boot. For A) The filler to tank (8mm) copper pipe, and B) The 6mm tank to engine bay pipe. Looking at this, I think the best route is down through the boot floor on the drivers side, and follow the exact same route as the fuel pipes to the engine bay.
As far as mounting the tank is concerned, if you are clever about it, there is no need to remove the petrol tank, or the exhaust, to get the bolts in and torqued up. I've got locking nuts on the bottom of my straps, to stop them working loose over time.
Updates to follow - but I don't plan to lose any time now, so progress should be fast!