I must say I've lost track of all the changes. I remember 508s at Reigate, along with CEPs, NSE livery CIGs and 3582 even turned up. That was the VEP with the 4TC carriage.
The initial electric service in 1994 was hourly Three Bridges to Maidstone West. Initially it was always VEPs, one Central, two South Eastern if I recall correctly. In May 1995 they added the Victoria to Tunbridge Wells trains. This was when most diagrams switched to CEPs and I think South Eastern covered the whole service. The Tunbridge Wells trains went all sort of strange routes between Victoria and East Croydon but mainly via Stewarts Lane and Pouparts Jn. They only stopped at East Croydon, Redhill, Tonbridge and High Brooms. Peak trains ran from London Bridge. Other stops were added over the next few timetables and I think they switched to London Bridge all day. The peak Reigate trains were tied in with this service.
This is where I lose track. At some point, presumably September 2002, the South Eastern trains became part of the standard half hourly stopping service between London Bridge and Redhill, XX:03 to Maidstone West and XX:33 to Horsham. These were booked 508s but 3CEPs often substituted. South of Redhill, the path of what would have been the XX:03 London Bridge to Horsham was taken up by the Tunbridge Wells to Horsham train. The latter route was a mix of Central and South Eastern CIG/VEP units. One turn was booked a South Eastern CIG which did the 10:32 and 13:32 off Horsham. They could not be CEPs because Horsham crew did not sign them. Throughout this period there were still a few late evening trains to Three Bridges worked by 508s.
As you say, in December 2008 the route switched to Southern and went back to hourly after 13 years of a half hourly service. Initially through to London but more recently just a shuttle from Redhill. The fortunes of this route have changed a lot in 30 years. Hourly, then two hourly, back to hourly, then half hourly and now back to hourly. Also three different regions/TOCs have provided the service.
The 508s were dire during the Connex era. Usually filthy and not very reliable. South Eastern turned them around. Ironically the final six were not long out of overhaul when the were withdrawn. On the last day I covered all four diagrams and all available track including the morning trip to Strood. It was a bitterly cold day with a heavy frost overnight but all four units were warm and performed faultlessly.