Thanks Spruce.
I would prefer to go upright.
Thing is I don't want a complete system as I have a static tank with RO etc...
You just tell them what you want down to the pumps and controllers and get them to give you a price. You will either have to phone them or send them an email.
I have got a 650 litre upright and I have experienced no handling issues with my Citroen Relay van. No you can't go around corners as fast as when the tank's empty, but I doubt that it will be any different with a fully filled flat tank.
When the tank is full of water we drive slowly and responsibily and don't through the van around sharp bends on country roads. The son does and you should see how his tyres wear.