One of the remarkable facts, at least to me, was the number of ordinary people who collaborated with the Communist authorities.
The percentage in Poland was about the same as everywhere else across the eastern bloc. I'd be happy to have a fairly large bet that that percentage was pretty much identical to the one the UK would have seen if the commies had ever got into power there.
When I asked my Polish friends they were very tight lipped and did not wish to talk about the matter at all.
You were lucky that they didn't want to talk about it: there are few things more pathetic than watching Poles try to claim that hardly any Poles joined the party (especially if said Poles go on to claim that most people in the party were Jews anyway). It is a sore point with a lot of people in Poland, probably more of a sore point than in other countries in the former eastern bloc.
membership would be required to get and hold ANY supervisor type job position
Sorry but that very simply is not true, at least not for the '60s, '70s (i.e. when the party was at its peak) and '80s. I know more than a few people who had senior supervisor positions who were most certainly not in the party. I know people who had senior positions working for Poland overseas who were not in the party.
sometimes to get a flat or a coupon for a car or other benefits, would help get passport or holiday package in Bulgaria.
That's a very strange statement; I know lots of people who got a flat and/or a car without needing to join the party. I very much doubt that being in the party hurt one's chances of getting those things but it was not necessary by any means.