Dazmond you alrady had a back up. Battery and pump.
All you need is a switch and put a fuse between battery and pump.
Also use one of those 1/2 inch pipe valves (that cost around a fiver) between the water tank and pump the regulate flow.
I stopped using the 1st varistream because the flow varied depending on the time of day and the time of year.
I originally had a teeny doubt about blowing the flojet but never yet blown a fuse. I also got 4 years out of a 90amp leisure battery getting charged weekly most times.
Tank > pump > battery is very simple.
I've read other people (who's opinions i take on board) on here who have done the same but are now using a flow controller and they say it is better than without. I believe them but i'm happy without.
All for future reference of course.