Err no...
but as you've been benefiting from an error that has cost Virgin £1800 per year, maybe you should consider sending them a cheque for £1800 per year for however many years you have been travelling.
Or alternatively you could accept it was an error and that you'll now have to pay the going rate
Or perhaps the "error" is just a way for them to avoid admitting that they're jacking up the price.
I certainly find it curious that this "error" has been around for four years without them noticing.