Thanks for these responses. To answer some of the questions, roughly in order:
1. I have not tested against any other 2.4GHz yet, but Giant Cod say they will do this. I would be amazed if I was the only person who had received two sets that interfered so obviously. I did, however do a range-check, albeit in a residential area, with only one R/C system running at a time. With the transmitter in my back garden, I walked out of the driveway with a small model boat, with ESC/motor running at "idle", and continued along the road until the motor stopped and the ESC bleeped to indicate loss of signal. With the Fly Sky, this was achieved without glitch or spurious motor response. The RadioLink, installed identically, gave a similar ultimate range, but for the last 20% of this, the servos glitched wildly and the motor revved and stopped, indicating susceptibility to some other source of interference, perhaps a neighbour's WiFi.
2. I have not yet met any other RadioLink owners.
3. I discovered this interference issue within 1 hour of receipt of the RadioLink Gear! I probably should have bought a second Fly Sky Tx/Rx but chose the RadioLink only so as to have two different-looking Tx's to avoid confusion. While checking for the unlikely bonus that one brand might bind with the other (they won't), I noticed the RadioLink performing erratically....
4. The "rather poor" video was only submitted to illustrate the frequency of glitches. Testing-wise, it is the tip of the iceberg, because I was motivated to find some way that these two sets could be operated together: after many permutations of Tx-Tx and Tx-Rx distances, up to 50m, so far, my conclusion is that they can't.
5. When the Fly Sky is seen to cause the RadioLink to glitch, pressing its "range-check" button, to reduce its RF power, makes no difference to the glitching.
6. The ESC/motor combinations (one brushed, one brushless) that I have tried have previously been shown to work fine with 27MHz, 35MHz FM (not for boats, I know) and 2.4GHz radio sets, until combined with the RadioLink, and then only under 2.4GHz interference conditions.
7. I made Giant Cod, who sold me the RadioLink, aware of my problem on the day of receipt (13 March 2012). Unfortunately, although they have RadioLink T4EU and R7EH in stock, they do not have any Fly Sky FS-T4B with which to reproduce my exact conditions. I hope we will be able to arrive at a good solution that I can tell to the forum.