Emma Stansfield asks:

Who is this Dr Brian Cox anyway (Letters, 2 October)? The last I heard he was a professor.

He is indeed a professor, at the University of Manchester – but he has a PhD as well! Many reasonable people have therefore chosen to address him as “doctor” – including The Times, who could reasonably be expected to be more traditional about their use of titles than the Guardian.

It’s true that Cox’s Twitter account is under the username “profbriancox”, but Twitter usernames are as yet neither legally nor morally binding.

Don McMahan asks:

Hugh Fearnley-Whittingstall calls for unsalted butter and salt in two of his fennel recipes. I’m sure a lot of people think there must be a good reason for this, but it’s really a lot of nonsense, isn’t it?

Well, it’s a bit of nonsense – recipes say this sort of thing a lot and you can nearly always use salted butter, with or without extra salt, and cause little trouble.

But it is the case that different varieties of salted butter contain different amounts of salt, so if you think there’s a significant difference between, say, a quarter of a teaspoon of salt and half a teaspoon of salt  – then you really are better off using unsalted butter, because it lets you keep track of just how much salt you’ve popped in.