I have used this method for about three years now.
Previously the letters stated that I was resident in UK ‘within the meaning of Article 4 of the Double Taxation Convention between the UK and Greece’. I never bothered to check up on the meaning of that. The formal letter which I received last year was slightly different. It stated that it was a certificate of residence but would not enable me to claim benefits in another EU state.
I have never needed the letter to be stamped or legalised in any way for submission to the local tax office in Rethymnon by my accountant. The basic Tax Office stamp has been adequate and acceptable.
As far as the application for the certificate is concerned it is interesting to note since last year HMRC have asked for your property details in Greece. (One good reason to be grateful for not having an exact address to quote.) Despite the current suspension of Capital Gains Tax on property sales in Greece, any sale of property in Greece (not a main residence) will still be liable for CGT in UK. Without the Greek tax to offset that will become even more expensive. Things have certainly been tightened up.
NB Just found the covering letter enclosed by HMRC last year, it also points out you may be required to complete a self assessment tax return in the event you sell property overseas!