is there anywhere that shows which stations are valid. Although I am not doubting anyone, I struggle to understand how a Clapham Junction to London ticket is valid for east croydon.
Clapham Junction to London Bridge is valid via East Croydon because it is a
mapped route.
In addition, due to the fast, frequent trains between Clapham Junction and East Croydon, and East Croydon and London Bridge, it is often the fastest route and is indeed suggested as such by National Rail Enquiries.
http://ojp.nationalrail.co.uk/service/timesandfares/CLJ/LBG/tomorrow/1300/dep
(Check the one change option)
As it is a season ticket valid for Clapham Junction - East Croydon - London Bridge, it is therefore also valid for East Croydon - London Bridge, as you may use season tickets between any of the intermediate stops between which the ticket is valid.
Also - I'm not sure this solves my original problem. Ideally I would like to travel from Reigate to London Bridge as this is the quickest route. Unless you are saying that i could use those tickets to go from Reigate via redhill to London bridge.
The train from Reigate to London Bridge calls at East Croydon.
A Clapham Junction - London Terminals season is valid from East Croydon to London Bridge.
This means you need a ticket valid between Reigate and East Croydon.
The Betchworth - Box Hill ticket is such a ticket. In this case it is valid because it is one of the shortest routes.
If you search National Rail Enquiries for Betchworth - Box Hill, specifying via Purley
http://ojp.nationalrail.co.uk/service/timesandfares/BTO/BXW/tomorrow/1400/dep?via=Purley
it offers
Betchworth - Reigate - Redhill
Redhill - East Croydon - Norwood Junction
Norwood Junction - Sutton
Sutton - Box Hill
Again, as a season ticket, this is valid via all intermediate stations, so is a valid ticket for Reigate - Redhill - East Croydon.
Since you have a valid ticket for Reigate - East Croydon, and also East Croydon - London Bridge, you therefore have a valid ticket for Reigate - Redhill - London Bridge