Thank you all for your answers.
I have gone to the financial people of my department and they are going to find out where my stipend
money comes from. They made it sound like it was a very definitive amount that I'm getting and that
not much can change.
I'm not very much convinced.

I'm going to find out what my minimum amount of stipend per month is as well, maybe there's a way
to get a bit more.

Ok these are probably very basic questions, but I have looked on this forum and couldn't find the definitive answer I need.

My supervisor gives me a stipend of 1000 per month for my PhD position, and he claims that is the maximum amount of
money he can give me, otherwise someone has to pay tax (either department or me). Is that true?
This forum suggests that stipends can run up to 18k per year without any tax, and this site
http://www.hmrc.gov.uk/manuals/eimanual/EIM06265.htm
claims that there's no tax at all for stipends of PhD students.

So basically: is there or isn't there a 'no tax' limit to a stipend for PhD?

thanks