At the end of my 4-year undergraduate course, I'll have (hopefully!) an M.Sci in Physics. I've read lots on this forum about needing a Masters degree, which I take to mean a postgraduate MSc.

If I have my undergraduate masters degree, will this be enough of a qualification to do a PhD or will I need to do a postgraduate degree first?

Well, officially, all you need is a 2:1 or above on a bachelors course. An MSci would be better and a postgrad MSc or MRes would be better still. Start applying for PhDs in November/December, if you don't get anywhere, you can always apply for a postgrad masters and do a PhD after that, but that's an expensive route

What is a Msci? Is it a masters degree?

A MSci is ideal and counts as a masters degree for applications. You shouldn't need any other qualifications.

Many thanks indeed - thought that would be the case, but it is often unclear.