I was in the school CCF around the time that the Hairy Mary uniform was being phased out of RAF use; we wore it until I left in 1979 and ALWAYS wore pyjama trousers underneath. I can confirm that in hot weather it was near unbearable, and even worn with pyjama trousers you could feel the prickliness. That stuff was also near impossible to iron and whoever thought it would be suitable as a clothing material must have been a sadist or an idiot.
By the time I got to Swinderby in 1980 thankfully the Thunderbird jacket and pseudo-lightweight trousers were the norm, though even those trousers were uncomfortable. I continued wearing pyjama trousers with my No.1 uniform until the day I left.