Thank you for the pointers. I searched other forums also and found similar products mentioned. The Desivent DV300 seems to be a common product, badged by numerous people.
I need to decide whether I should buy a couple of 0.5kg bags of self-indicating silica gel and just swap them out when I fly (and put the used ones in the oven at home) or buy a couple of DV300s.
There is no data anywhere on the DV300 and my guess is that it contains only a very small amount of silica gel. I use the stuff at work for packaging certain electronic components and it needs heating to about +120C to clear it out, and I can't see the whole DV300 unit heating itself up to +120C...
Also some of the self-indicating stuff looks rather toxic.