wrote:

I'm setting up a water efficient garden, so I'm buying a large tank
(4,500 litre) and also using a number of drums under the downspouts
around the house.

I used to fill a 500 gallon tank which was O/H with a "wobble" pump.
The larger the pump the quicker it fills, Duh.
I don't know if they're still available new, second hand they should be
laying around somewhere. Post a requirement.
Good exercise early morning! Change hands!!!!
fluff