This may be a silly or naïve question but...

If a garage quotes 21 hrs labour for a job does that mean that the job takes 21 hours work to complete it (about 3 working days) no matter how many people are working on it - or is it for costing it so that 21 hours might be two blokes working for 10.5 hours or 3 blokes working for 7 hours (or, rather unlikely, 7 blokes working for 3 hours)?

 

It would be 21 hrs in total, no matter how many techs are working on the problem or how the time is divided between them.
I Here in the US, they call it "book time," which is a standardization, what they estimate the job should take a well equipped and trained tech. It's often the safer way to go on both sides financially speaking. The repair center can offer you a quote that is closer to exact that way. If it takes them longer, you still only pay for book time. But if the tech is well equipped, trained and experienced, he/she can often get the job done in less time, so it works out in their favor because now they can move on to the next paying job.
It's like how insurance works. Some people will never put in a claim but will still have to pay premiums, while others may have multiple claims that financially exceed the premiums they have paid.
See?