The Greek-UK DTA was written back in the 50s. I doubt if anybody involved in constructing it, on either side, is even alive today. The wording is somewhat arcane and I doubt if most Greek tax inspectors have ever read it. Greece has DTAs with many countries. In a large number of DTAs "public" pensions remain taxable in the country of origin and "private" pensions are liable for taxation in the country of residence. The email from my accountant had the following statement, my highlight in red:
"The favourable tax regime is applied to pensioners in the private sector, as public pensioners are usually taxed in their home country. In any case, government sources say, double tax avoidance agreements will be observed.
So does the UK fit in the "usually" bracket or not? Does the tax inspector dealing with your return know what the DTA says? This sort of confusion is common in all tax regimes. I know people who have been told things by HMRC staff that are the exact opposite of what it says in the Greek-UK DTA but probably were in line with the bulk of the DTA's they dealt with. Even their own internal guidance document has errors in it.