What's the best way to change Euros back into Sterling without losing too much in fees etc?

I've just arrived back from Spain, and as usual still have some Euros left over - normally I would just keep these for my next holiday, but due to winning a bingo jackpot Wednesday night (what can I say - it was raining so I couldn't go anywhere!) I have rather more than usual. I've just been onto the Post Office website and worked out that if I 'sell' my euros back to them I would get �55 less than what it would cost me to 'buy' the same amount of euros from them in the first place. Any suggestions?