You don't have to have the one i've got NWH, which is a rinnai, a morco would do the job too. You said you liked the idea that the L5's gas turns on when you need it and off when you don't. In fact they all do this, and that is why it is called an on demand system.
The brand leader puts another lpg fuel tank where your spare tyre is and feeds from there. Many garages sell it now at the pump.I have tried to enquire about the cost of this but can't seem to get an answer- mostly they want to do a complete conversion-.
I believe rough running costs per day are £2 lpg and £5-£7 diesel. You can use red, but who wants a thousand litre tank of that in the back garden.