The planned itinerary was 18:24 Theale to Reading, arriving 18:35, and then Reading to Camberley at 18:39
You mean this...?
Reading 1839
Ascot 1907
Ascot 1919
Camberley 1931
Although I believe the excess fare, once issued, arguably does entitle travel via Ascot, that is certainly not the intention of an OD excess.
So, your argument is along the lines that you want a fee-free refund of an excess fare which was to be used via a route for which, if it was known you would be taking, the OD excess may have been refused, and via an itinerary that isn't a valid itinerary as it doesn't meet the minimum connection time?
I don't think it's worth arguing over £4.60 (which is the value based on an undiscounted Anytime Day Return; you've not specified the ticket type or any discounts)
. The inbound from Theale arrived at 18:39 so the connection was no longer makeable. The MCT means that I have no entitlement to anything to my knowledge; then again, I’m pretty sure that a delay of any magnitude can allow a fee-free refund for choosing not to travel which might(?) apply in this case.
The question is whether a 3 minute delay on an invalid connection entitles to you a fee-free refund of a fare for a journey that was only possible on the basis of making an invalid connection.
If the following applied, you'd be entitled to it:
Your ticket is refundable (with no admin fee) if the service you planned to travel on is delayed or cancelled, and you choose not to travel.
I think it may be a bit of a stretch to say this fully applies in this case.
If it was a valid itinerary, then you would.
Delay Repay clearly does not apply; I wonder if they said that to fob you off. If Delay Repay applied, that would mean it was a valid connection, meaning they should be issuing a refund as you did not travel.
Unfortunately such nonsense from ticket office staff is not uncommon.