Thank you for your question. I cannot comment on the circumstances of the incident in Greece as I am not expert in Greek Criminal Law.
I can, however, confirm that the Extradition Act 2003 applies here. The Act created what is known as a European Arrest Warrant (EAW). The criminal Court in Greece may choose to issue one of these in the event that the offence is of a serious enough nature to require your attendance at trial or for punishment. It may, however, be that the offence is of a minor enough nature for the Court not to impose a custodial sentence in Greece in which case an EAW would likely not be issued. Without any experience of Greek criminal law it is not possible for me to say. Your Greek lawyer can no doubt assist you in this regard.
I can, though, confirm that there is a risk of extradition here. If an EAW is issued then the police and Courts in the UK are obliged to extradite you to Greece either to face trial or, if tried in your absence, for punishment.
With that in mind it is in your interests to be represented at that hearing and for the purposes of any trial that may come. I cannot say whether the 500 Euro charge is a reasonable one and you would have to take this up with your Greek lawyer. Unfortunately, though, if you choose not to make the payment to the Lawyer you will not have any opportunity to have your version of the events put forward.
In any Court hearing there is a risk that, if you are not present or represented, you do not have any opportunity to influence the Court's decision. As such you may think it worthwhile to make the payment to your Greek lawyer in order to ensure that your version of events is represented. That is a decision for you and I am afraid that I cannot advise you one way or the other on that.
Please do be aware, though, that there is a risk of extradition here. As a result you would do well to have some sort of representation in Greece.
I appreciate that this is probably not the news you were hoping for but it is important that you are aware of the risks. Good luck - I hope things work out well for you.
Please do not forget to rate my reply so that I can close off your question.