" Hi guys - thanks. Baz - an outside tap is just not possible"

I do not agree that a poly tun is a temporary structure( as somebody
commented above) and it should therefore have all necessary
services done proper !

Mains water would be the simplest, but not necessarily the cheapest without
rainwater supplies
augmenting supplies. You should consider T- ing off the existing underground
pipe at some convenient point
on your land and taking a supply in standard blue feed pipe underground to
the tunnel . A buried stop cock should be incorporated
just before the pipe rise in the tunnel for winter isolation and servicing
purposes. The "plumbing" for this is simplicity itself.

I would always consider mains leccy to be an asset as well - the overhead
method is deffo then best and cheapest.
Not always practical though agreed. Burying cable can be a bit of a pig -
regulations part xyz and all that !!

If you go down the road, of rainwater storage supplies then a pond pump is
perhaps the best means of aiding propulsion,
as water storage height factors are considerably reduced.

Regards
Pete - Nanneys Bridge Nursery, Cheshire ( retired )