FarmI wrote:

???? The time to refill will depend on the size off the pipe. As I wrote
earlier, if you use a quarter inch drip line it would take a long time. If
you use a half or one inch pipe it will take minutes.

Okay, I was thinking about retaining the existing inlet regulator, which
is 1/2" pipe and designed for mains pressure.

What I think I might do is look at just putting in a trough float arm.
This was effectively what the original toilet had until it wore out and
was replaced by this self contained device.

So, if I can fit a simple trough float arm in there as well, I could
just run a parrallel pipe from the tank at ground level. So, when the
tank water level drops too low, then I just turn the mains tap back on.

Shouldn't be a problem with the newer plastic walled cistern, but might
have to come through the bottom on the older ceramic cistern.

Problem solved.

We have 1,000 & 5,000 litre tanks atm.