Rite of passage yesterday - the first time (in nearly 5 years) I forgot to "press" the Senior Railcard "button" on the ticket machine screen.....must be getting old and forgetful
You will be able to get the difference back if you haven't already made a claim in the previous 12 months. Supply evidence to the relevant TOC▸ (Train Operating Company). There's an old MSE article here (doesn't look to be many offical TOC pages detailing this - but can confirm I have successfully got my money back in the past): https://www.moneysavingexpert.com/news/2017/03/forgetful-railcard-users-now-able-to-claim-refunds-if-theyre-told-to-pay-full-fare/