There used to be not one, but two agencies which were part dedicated to channel crossers, one from Lydd Approach (in the days when they had full time instead of part time ATC) and the other from Kent Radar (Manston) which was a civil element provided by NATS working in the RAF Approach Radar room. I think Lydd only had a Decca 424 so they were unable to provide radar service to crossers, whereas Manston used to have an AR1 (could have even been a 1b) followed by a Watchman.