Thanks to all for the ideas. Anyone used Sigma with any positive results?
I use the Sigma 506. I bought it for three reasons. One, after a Ciclosport HAC4 broke the day after the guarantee ran out, costing me about a 100 Euro a year for the three years it lasted, I wanted something reliable and the most reliable bicycle instrument I have ever owned turned out to be a Sigma PC9 heart rate monitor on another bike. Two, all Sigmas including the least complicated have autostart whereas comparable models of Cateye don't; this is such a basic convenience that I really don't want to be without it. Three, I went off Cateye after in little more than a year I had to spend near enough 200 Euro on blinkies that fell apart or proved to be inadequately waterproof.
I've also found the ergonomics of Sigma instruments excellent. The 506 for instance has a single large button that operates all functions. and it also has autoscan which essentially means you don't bother to operate even the big convenient button unless you switch off the autoscan, and even the five buttons on the PC9 HRM are easily operated while wearing dress gloves. All Sigma known to me have larger digits than any Cateye I've ever seen.
Note though that the inexpensive 506 is very basic indeed: it doesn't show average speed or maximum speed; it shows current speed, trip distance, time elapsed, total mileage, present time clock; it does either kilometers or miles. I wear my old flying watch as a cycling watch anyway, so I can compute these matters on the slide rule bezels if I want more precision than available through mental arithmetic. But, based on my experience of the two Sigma instruments I own, I have no hesitation recommending a Sigma, even if you go for something a bit more completely specified than the 506.
Sigma also offers a feature worth having on even their basic models, whereas Cateye doesn't offer it until you spend real money: this is the ability to start the odometer (total distance) from a non-zero number, a capability I consider essential for anyone with more than one bike and also when you put a new computer on a bike which already has some miles on it so that you can, for instance, keep track of service intervals.
From the very complete Ciclosport HAC4 the features I actually miss on my current Sigma 506, which cost only 5% as much as the HAC4, are surprisingly only two. There's the average speed you mentioned (not a big deal unless you're in training but useful all the same) and max speed recall, which isn't actually useful but which I like to know all the same as I have some good downhills. At first I also missed the altimeter functions of the HAC4 but now I just enjoy the ride.
The 506 and presumably the other Sigma too have a design where the mounting is just that, a mounting; some bike computers radio or wire the information to the mounting and then transfer it to the computing head, so that the mounting itself is valuable and gets stolen (there used to be a flourishing trade in expensive HAC4 mountings on Ebay). An active mounting is something else to go wrong or let in water. Sigma guarantees the supply of spare parts, and the mounting is cheap.
By the way, the mounting itself is light enough to bend under force. I discovered this by accident and then used it as a "feature" by bending it open to fit over the big black plastic ring of the Rohloff twist-shift, so that the computer doesn't take up valuable space on my handlebar or stem. Another good place to mount the Sigma computers, made possible by the halfcircle mounting design, is on the brake clamp ring to the handlebar. (There, in a crash the cheap computer gets trashed, not more expensive components.)
You can see I'm impressed and think I got value for money from Sigma. Their 506 is altogether a cycling computer (and the PC9 HRM too) designed by cyclists for cyclists.