A few screws and I removed the casing, and found the problem - for some reason there was a bit of resistance in the actual switch. An eye-dropper of light oil, and it's completely free, and working as usual. It's easy to test, as the moment the water container is moved, it cuts out (you get a red light on the panel). For good measure I fitted new filters too.
However, since this, and resiting the unit, it runs as before - the humidistat seems to cause it to cut in and out. However, 4 days later, there's not a drop of water been collected in the container.
The only hint as to something is now when it's running, there's a click sometimes - like a relay somewhere is cutting in or out. Sure it wasn't there (or audible) before. It's not the switch for the fan, as the fan keeps spinning through the click.
After checking the filter and humidistat (both done) the manual goes to "request a service". This being DIY, I wondered if anyone had any ideas first ?