Okay I've finally snagged an old DVB-T dongle buried in the warehouse and it's identified as having a:
"Rafael Micro R820T tuner"
And, lo and behold, it gets plane tracking results almost immediately, even in my sub-optimal desk-bound conditions.
This is obviously quite starkly different to the new ones. It looks like the tuner part of the chipset has been switched out without us knowing. Presumably for perfectly sensible reasons; if you consider that it's supposed to be a DVB-T dongle and still functions as such, it's hardly surprising.
However, since we sell it expressly for plane tracking, and advertise it as such, we'll obviously find some way to put this right.
As for your return, would you like a refund for now, or a replacement? I can't make any promises when we'll get any compatible dongles in, but I could have a quick dive through our remaining stock to see if I can locate any with the old tuner part and verify that they will work.
Edit: I've found and tested another dongle if you go the replacement route.