Gas boilers.

Hi all im coverting my beloved county into a camper and im a bit miffed about the whole gas regulations and how to generally how to go about it all. Ill be running two 9ltr carver 2 boilers and my cooker appliances and i have several questions...

1. Can the gas bottle be stored underneath the van if its in a metal box, is this wise, what are the regs on this?

2. I dont want to cut holes in the sides of my van as its got some nice check plating for the outlet, Can i use flexible ducting for the exhuasts from the boilers instead of double lined ducting? Again what are the regs? Is it wise as they will be in the walls?

3. If I install it all myself can I just get someone to check it after to gain a certificate?

Any help is greatfully welcomed and pics would be handy too...

Thanks in advance.

J