Right then...
Basically, Harvey's covers a range of brands of the non-electric softeners including TwinTec, Harvey's, etc with are all effectively the same unit. Kinetic I think is another brand that does non-electric.
If you aren't up during the night and have easy access to electric then I would always recommend Tapaworks. They're much cheaper than non-electric. If you don't then go non-electric but a refurbished model (old plastic case, new internals). Then pay a plumber to come and fit. If you go with one of Harvey's new then you'll be paying £1.5k including installation. It's a giant monopoly and none of the installers will undercut their rivals.
Non-electric tend to use block salt which is more expensive.
I went for the refurb route yesterday and it cost £950 all in.