Thanks for the comments. And apologies for not researching this topic
more fully by googling this group before posting. Here's the results of
my few days' research.

1. Most people think these devices are ineffective and a scam
2. A few people argue that they are effective at preventing limescale
formation (yes, even from experience)
3. There is a dearth of real-world testing to prove the point either
way. Whatever they do achieve, though, they certainly do not soften the
water.
4. Whilst it is quite possible to get an electronic unit with a full
money-back guarantee, I would be concerned that any damage to my
plumbing (by way of scale build-up) would not be covered, if I ever had
to return one of them.
5. My conclusion FWIW: these devices (if you get the right one) *may* be
some aid to the prevention of limescale build-up, but are definitely not
a substitute for genuine water softeners. I will be replacing my old
ion-exchange model with a similar unit.

Cheers
D