After going through three thermostats in as many years (admittedly the first one had probably lasted 34 years) I decided to bite the bullet and fit this aftermarket unit.
The original Kenlowe version as fitted by Theo to his Trevi does not seem to be available any more. This one was inexpensive (£19.99) and I do have some concerns about it's durability, but it was easy to fit, seems to work well and has a very wide range of adjustment. I've set it to come on with the gauge at dead centre, which equates to 95 degrees on the dial.
I don't like fitting non OE parts, but it seems to me that the original units are possibly deteriorating with age? Fortunately, as reported before, the last two failed in the 'on' position.