Category property and a few other of the Built-in properties are not culture specific. That is, you can only edit the vaule on the Master Language branch of a page.

I guess the reason you see the read-only icon is because you are looking at another language than the first created. Try switching language and see what happens!

The only language I have is en-gb. I've removed all other languages from both the admin and also the root node for the site as language option. The items still display the en link when going in to the properties view however but I've also checked the versions in and there is no link to delete versions of en, but currently it's all configured under one language

Oh, I'm afraid that there is a risk that your database is in a bad state now. In worst case you have to fiddle directly in the database to recover depending on how many pages you have created with different languages. 

How many pages do you have created using "en" vs "en-GB"?

We swtiched to en-gb right at the start of the project so there's unlikely to be many en version of pages kicking about. I'm going to re-enbale the en version and see if there are any phantom versions knocking about, else a database job is likely I'd imagine, we have too much content really at this point to start again.

It seems there were indeed version of EN that I couldn't delete and no matter what I looked at doing it wouldn't work. I got round it by creating a new website with the correct language then moving all the items over, this site started in en-gb so had the correct master language set and therefore all the core properties are now working, the only alloying thinking is looksing page links in my pages and blocks but a little manual work should fix it, at least the whole site is running in en-gb now. Thanks for your help Fredrik :-)