Firstly I'm in Scotland so water metering is not a consideration. I've considered using rainwater recycling for the tunnel, but will be faced with a soakaway to cope with the winter rains so I see no point in putting in a tank and a soakaway.
To get at a mains supply is going to be awkward, but if my memory of the bathroom underfloor configuration is correct, not impossible.
What I need guidance on is what sort of piping to use. The run to the tunnel will be about 10m underground from the house wall - there is I think a suitable hole under the wall of the house (it's a stone cottage, 3ft thick walls and no founds) which will give access under the bathroom where there is a mains supply. I can just squeeze into part of that area under the bathroom floor, but will need a pipe with maximum flexibility to get it through the hole under the wall, under the bathroom and then back up under the sink where there is mains.
Many thanks for any help - all the plumbing I have done in the past is in copper and I've never used plastic. It's probably ideal for this situation but can I use just one type of pipe for the whole run?
Rob