Using Thameslink ticket on Southern

Wondering if someone could help me with what actions I should take in regards on the below:

I currently hold a Monthly Thameslink Only ticket from Brighton to London Victoria including Zone 1-6.
I have used the ticket for over 2months travelling from Hove to London Victoria on a Southern train and never had issue until today when the ticket inspector told me my ticket was not valid and charged me £25 for another ticket on Southern.

I have read the National Rail clause that some other posts on this site had mentioned, about this not being correct due to the ticket not being able to be linked to a specific brand e.g. Thameslink or Southern.

Could anyone advise on the correct way to go about making a complaint and potentially getting a refund on this charge?

Thanks in advance

This is a 'Dedicated' fare (in accordance with the Ticketing Settlement Agreement) and is valid on trains operated by the named company, in this case [Govia] Thameslink.

What you need to do is write to Govia Thameslink and ask the company to refund your £25. There is no separate company called Southern and hasn't been for several years now.

However some of the ticket inspectors who work for Govia Thameslink act as if they are working for a defunct company and claim that the tickets are not valid. They're just doing what they're told, as the company doesn't want the cheaper fares to be valid on all their route brands. But the company doesn't want any legal hassle over this, so they do refund people who ask for a refund, in my experience. In fact I am not aware of anyone who has sought advice here and subsequently been refused a full refund. So I'm pretty confident you'll be successful.

Feel free to send me a draft of your letter to the company asking them for a refund; I will be happy to proof read it.

Until there is any further development, there isn't really anything else to say about this; the topic has been covered in numerous threads before, if anyone else wants some reading material about it, there is plenty here: https://www.google.com/search?safe=active&q="thameslink+only"+site:railforums.co.uk