Have a look at EHICPLUS, which covers single trips up to 122 days. It is not one way Travel Insurance, so may not be any good, if you don't know the date of your return to the UK.
World-first.co.uk are Travel Insurance specialists and may be worth speaking to. PJ Hayman also provide many different options and would be worth speaking to.
Have a think about what cover you really need. If you are taken ill whilst staying in Greece, are you happy with the healthcare arrangements there, given the state of the Greek economy. Remember that the normal EHIC only covers state health services and if you need private treatment or repatriation to the UK, it won't be covered. This is where Travel Insurance comes in very useful.