I used to fly a Cessna 150 with a similar problem, only with Humberside.
At the time, Humberside was the only radar in the area that derived it's SSR from a remote head, from where it was transmitted by phone line to LATCC where it was processed and sent back to Humberside, again by phone line.
Never really got to the bottom of it, but we put it down to the combination of an old transponder producing data incompatible with the transmission and processing involved in the new system.
MJ
