This is why I love this forum so much ... there's always a great cross section of information, help and advice from everyone sharing real-life experience.
A BIG thank you to all for your feedback, it really has given me ideas and info on the options available and things to look out for.

Out of curiousity I'm taking my van to our local gas installer to have a sit down chat with him next week. I had a brief chew on his ear yesterday, he's an old school mechanic who's been doing gas instals here since 1971 and seemed to know exactly what I was asking about, has the parts, has the knowledge, has the tanks etc, etc. He reckons he'd fit an 80 litre tank in for dual and can add a second 40/50 litre one in where the petty tank goes if I went straight gas.

Thanks again to all for you advice, it's been a great help. cheers Tony