I could have added that another complicatng factor at SEN is the gen avn traffic in the circuit when Easyjet perform overhead arrivals when the SSR is u/s
Not familiar with the EZY overhead arrivals but (a) base of controlled airspace is 3500ft so any EZY arrivals are presumably at 3000ft; (b) visual circuit height is 1000ft; (c) all the aircraft concerned are subject to air traffic control - the circuit traffic is within the ATZ and the EZYs will be on a Traffic or Deconfliction Service; (d) you don't need SSR to positively identify traffic and give it a Deconfliction or Traffic Service - "Easyjet 1234 traffic in your 12 o'clock five miles, VFR circuit traffic under my control not above 1000ft".
Southend operated entirely without SSR until fairly recently.
NS