Its not a Swanwick fault
This isn't a Swanwick fault but a fault with the Radar. This has been proven by looking at the raw radar data before Swaniwck even touches it.
From what I understand is that a particular Radar is sometimes having problems decoding the transponder signals from aircraft flying a particular route in the North Sea Sectors.
Why does Swanwick use the bad FL?
The radars should filter out FL which show excessive rates of change - 500 ft in 6 seconds is probably considered excessive. The Mode C would still be passed to Swanwick but with the invalid Mode C bit set. This isn't happening so Swanwick thinks the Mode C is good and uses it.
Why not just use Single Radar Mode in this area?
You would still get STCA alerts as the ALP processors use Multi-Radar tracking, as its better for lateral tracking.
Why was this not spotted during Swanwick testing?
1) Its a Mk 1 eyeball test using real live radar feeds, easy to miss especially if it only happens every now and again.
2) Most of Swanwicks tests were done with manufactured radar data which doesn't have this problem