My flow controller went pear-shaped on Friday morning, so I fitted a small switch to it (I had got this from Wilkos a long time ago, just for this kind of problem), so I could turn it off and on; using the pump's pressure switch for the rest of the day. It meant working at 'full blast', but that's fine by me.
And I've just fitted a spare controller that I had ready made; it took about ten minutes to connect it up.
No downtime.
I'll order another two volt regulator and another two switches and re-make the one I've just taken out of the van.
It's very simple because the boxes already have the correct holes cut in them and I just re-use the wires; they're already cut to the right length.