Paulboy,
First fix is when you fix the back boxes and run in all the cable but nothing is connected. The plasterer then comes and does his bit, unimpeded by sockets and switches. He gets an easier job, you get a better finish. Then we return for second fix, where we connect up all the bits, make everything work and issue you with a certificate.
Doing first and second fix in one go means that you only see us once and the plasterer has to work round all those switches and sockets. Plasterers don't like this, but there are two approaches:
A good plasterer will patiently work around, removing the plates and making sure the plaster gets where it's supposed to be. It will cost you a bit more, because it's a p.i.t.a. but it needs to be done properly.
A shoddy one (and sadly, there are plenty of them) will simply plaster up to the switches and sockets, leaving them at whatever angle he found them, burying sockets sunk a little too deep, scratching the surfaces, with no regard for getting a flat or even finish to his own work. Then it looks like you've not only had a rubbish plasterer, but also a rubbish rewire!
Yet another reason to get it done before you move in.