First of all congrats, a Diploma from NTUA is no small achievement!
A careless recruiter (or a not so bright keyword matching software) might confuse your degree for a lesser one, so you should make the years of study abundantly clear. However keep in mind that Diploma (or various forms of) shouldn't really confuse the average recruiter, several other countries' universities use it as the title of their degrees.
Most resumes I review are the other way around (degrees from non Greek universities), and the more common way I've seen them clarify degree equivalence is:
Years of study: 5 - Equivalent to a M.Eng
And since NTUA's website offers most of its content in English, it wouldn't hurt if you included a link to your school's courses page. A link to the specific flow you followed, noting perhaps that this is the part of your degree that's (roughly) equivalent to a M.Sci would be advisable as well.
A possible structure of your resume could be something like:
2006-2012 - Diploma of Electric and Computer Engineering
Degree equivalence: M.Eng in Software Engineering (minimum years of study: 5)
Flow: Signals, Automatic Control and Robotics1
As Drareg mentions in a comment, you should look at Europass and you should ask your school to give you a Diploma Supplement:
A document describing the knowledge and skills acquired by holders of higher education degrees. Examples
It provides additional information to that included in the official degrees / diplomas and/or transcript, making it more easily understood, especially by employers or institutions outside the issuing country.
Another thing you should probably avoid is calling your school a Polytechnic, we may call all our engineering schools Polytechnics, but in the UK it means something else:
A polytechnic was a type of tertiary education teaching institution in England, Wales, and Northern Ireland offering degree and post graduate education that was governed and administered at the national level. After the passage of the Further and Higher Education Act 1992 they became independent universities which meant they could award their own degrees.
UK polytechnics might be considered universities today, but back when I was in the UK they carried a stigma for being lesser schools and using the word might raise some eyebrows.
1 Picked the one with the coolest title, as an example ;)