Hi Dave,
This probably isn't a lot of help to you. The second Shurflo pump I have on my system is a 4l Shurflo. I brought it from Williamson's about 5 years ago as I was experiencing pressure switch problems with my 5l pump which was cycling.
It worked fine to be fair without a flow controller, but in those days we used brushes with 3mm jets. Water consumption per day was a real issue as I had one of Peter Fogwill 375 litre trailers and an additional 100 litres in drums in the van would get me to about 3.30pm.
When we changed to brushes with 2mm jets we immediately noticed the 4 litre pump was cycling and so went over to Varistreams. In those days I preferred a flow of about 1.5 lpm and the Varistreams helped reduce my water wastage. I would estimate that my average day's water saving was between 75 and 100l. I started to use Minibore at the same time and have about 80m on my hose reel.
The reason why the 5 litre pump was blowing pressure switches was that it was in a pump box that filled up with water when an internal pipe sprung a leak. I stripped it down and let it dry out in a warm spot for a few weeks and put it back together. Bought another Varistream and it's still working today.
If I were in your position, I would buy a Varistream / pump / filter kit and replace the lot. The messing around trying to save a few pennies can cost dearly with downtime in my experience. You can still work in the meantime, maybe do work closest to home so you can top up through the day (if you use an IBC tank) and at least get something done.
I know there are guys out there who advocate a 5 litre pump will supply the perfect amount of water using 100m of micrbore hose, without the pump cycling, so I can only talk from my experience.
Spruce