I remember them well Oh Slippery one !!
When employed I was Approved Grade , there were weird rules like ,you would run jobs with 5 Electricians ( or something!) at your normal hourly rate , but if it was a bigger job with more labour your rate went up by a shilling (5p) ( was it ? ) The responsibilities imposed by the firms made it not worth the money ....although it was a learning curve.
I came across Tech grade sparks working at Approved rates all the time as no one would employ them because of the (then) high rate.
Union rules Eh ! The ETU ( the one run the Communists ) and then the EEPTU didn't think much of us contractors I'm afraid ,as we were difficult to organise ......... the sparks employed in the big industrial plants and car factories came first .......we were nobodies to them .