DERG, things become clearer, thank you. As you came from a communist country you were therefore barred. The following extract, although 3 years after your date will illustrate the point. I would guess that he had joined well before 1968 and had been lucky to retain his clearances.
We had many Poles that has served in WW2; they were all, as far as I know, put in non-operational appointments.
In June 1971, a Czech-born former RAF sergeant was convicted of espionage on behalf of the Czechoslovak intelligence service, which is known to have passed on information supplied by him a about the V-bomber force to the USSR. [19] The spy concerned, Nicholas Prager, alleged in court that his wife, Jana, had been seduced by a Czech diplomat in London whom she supplied with secret papers.