All right, thank you both for the information, which I will pass to the migration agent and my employer. In fact I think the rule you are referring to is that you cannot have another employment _at the same time_ - and I don't, the UK and Australian jobs are in different parts of the year. At https://www.border.gov.au/Trav/Visa-1/457-
you can read that:
This visa allows you to:
•work in Australia for up to four years
•bring your family to work or study in Australia
•travel in and out of Australia as often as you want.
which does not imply that the Australian employment has to be continuous.
However, whatever the outcome of this issue, the question still stands - my 457 visa expires on the same day as my Medicare enrolment, shortly before my next visit to Australia. You cannot renew Medicare from overseas, it seems, so our migration agent getting our 457 renewal application together tells us we need overseas visitor health insurance. Anyone know a way round this? Or if not, can we take out insurance and immediately cancel it once our Medicare is renewed after arrival in Oz? Do the providers differ in this respect?