I've been looking at It Always Rains on Sunday in the light of the conflicting comments on offer. In case it's of any interest the situation seems to me to be as Elvis has it: Doris wears a plastic mac, a sort of pearlised white, and Vi, the tearaway - snogs the record salesman in the audio-booth - wears a clear plastic mac. There's another girl, partner of said salesperson, who is seen once or twice, and who is wearing something in tartan. I guess it is this that is identified by some observers as a rubberised cotton mackintosh. I can't myself confirm the sighting - my copy of the film is probbaly been cut and I'm not seeing the full evidence. But from what I have seen it could well be rubbersiecd cotton, if only because tartan macs in that material are to be seen elsewhere. For example, Dirk Bogarde's partner wears one as they sit in the rain and do the miserable lovers bit.
One of the great things about It Always Rains on a Sunday is that both Doris and Vi not only wear their macs quite a bit - the eponymous rain is eponymously ceaseless - but they put them on and take them off with absorbing frequency too.