Going to Halkidiki at the end of June and not sure whether to get our euros now when the rate is up and chance that they don't change the currency or leave it until closer to the date. Any ideas?

You are going to have to get something other than Sterling, I would get €uros and change them on a day to day basis if Greece has gone back to the Drachma by the time you get here, then if you have any left, you can spend them in Germany at least !!!