Consider putting in what I did ....I have a 6500L underground storage tank.

All of my downpipes go to underground surface water drain as normal ...
what I have is after final gulley connection, I have the pipe diverted
to tank ... so it collects all roof water.

I added a large outbuilding last year and have also added that to take feed.

System is automatic .. as long as water is 10% in tank the system
provide mains pressure recycled water, if tank drops below 10%, it auto
tops up with mains water ... saving messing about with manual divert
valves etc.
I plumbed in a 3rd pipe run in house .. So I have Hot, Cold & recycled
.... the recycled goes to the WC's washing m/c and outside taps.

At least 30% of your water is used in flushing your loo .... so I save
well over that.

You can buy the bits individually, I bought it as a System from ...

http://www.freewateruk.co.uk/rainwat...g-smartfit.htm

Although I found after sales pretty dire, product itself is OK.
Been in use now for 10 yrs.