That's right - supererogation. I had to look it up too (it means 'spending over and above, beyond the call of duty'). And I had take a couple of run-ups before being able to say it out loud (think super + erogation).
A civil servant in 1948 would no doubt know this word, and therefore Gowers's sense of audience was impeccable. But these days we would tend to write about plain English in plainer English.
So I've had a go at translating a key section of Plain Words into plain English:
The original reads:
"A new technique is being developed for those pamphlets and leaflets that are necessary to explain the law to the man in the street in such matters as P.A.Y.E. and National Insurance. Its guiding principles are to use the simplest language and avoid technical terms, to employ the second person freely, not to try to give all the details of the law relevant to the subject, but to be content with stating the essentials, to explain, if these are stated in the writer's words and not the words of the Act, that they are an approximation only, to tell the reader where he can find fuller information and further advice, and always to make sure that he knows what are his rights of appeal."Proof, perhaps, that a 92 word sentence can be reasonably easy to read if it is simply a list separated by commas. Here's my version:
"How to explain things like P.A.Y.E. and National Insurance :We haven't really improved on that advice in 60 years. To try would be supererogation, 'nuff said.
- Use simple language and avoid technical terms
- Use ‘you’ as much as you can
- Don’t try to give all the details, but just give the basics
- Say where people can find fuller information and further advice
- Always make sure people know their rights of appeal."