Mine wasnt great Neil. Im getting 12-16 litres a minute at the house, as I have about 100ft or so of 15mm running from the meter to the house. Only solution for me to get more was a new main from the road down the drive. Luckily for us the 12l/min is just fine for the shower so we didnt need to upgrade the mains.
I can imagine with more than the two people in the house, then there may be the possibility of capacity issues at 12-16l/min.
If there is an issue then another solution would be to retain the tank/s, but use them to feed a 3bar 30l/min pump which in turn feeds the unvented system. it just mimicks a 30l/min 3 bar mains until such time as your feeder tank runs out, at which point it drops back to the 12l/min.
In effect if you are running off 30L/min and the mains is 12L/min, then you are draining your tank at 18L/min. with a 50 gallon tank thats going to take about 12 minutes to empty it. Or around 360 litres in that 12 minutes, or about 4 baths!