Basically, Jeff, when you get your state pension you ask DWP for an S1 Form. If you have a dependent partner ask DWP to put them on the form as well. You take the S1 Form, which is in duplicate, to IKA together with your passport, photocopy of your passport, residence certificate, and some passport size photographs and anyone of a dozen or so other documents that occur to you, e.g. ΔΕΗ bill, Cosmote bill and house/rental contract. They will issue you with an IKA book. When ever you go to an EOPPY doctor you take the book with you. Also worth taking with you if you go to a private doctor and they might give you a lower fee but that is not guaranteed.
Eventually, after a couple of millennia, IKA send the second part of the S1 Form back to DWP to prove that you have registered with them. Then you can apply to the Overseas Healthcare Team
at DWP for an application form for an EHIC. That EHIC cannot be used in Greece but can be used elsewhere in the EU including, I think, the UK.