Originally Posted by UKtravelbear

In total it would still be £536 but you will have paid a good chunk of those already in your PE ticket e.g. the airport fees are the same as is APD so it's really just the addtional VS fuel surcharge.

Easiest way would be to do a dummy booking for PE and the difference is the extra you would have to pay. I'd say about £ 100 but could be less.

This does assume that you have a S or W PE fare. If it is a K fare then you would have to pay to change your K to S or W which would be a couple of hundred £