Let's assume that your gas fire has a heat output of around 4KW. (That's not just a wild guess. It's taken by looking at the specifications of the gas fires shown here:
http://www.manchester...product/Gas_Fires/17/ ).
Further, let's assume that you turn it on (at 'full blast') when you rise at 7am and turn it off when you go to bed at 11pm. That's 16 hours at 4KW, so that's 64KWh.
Tariffs vary, but British Gas (on their standard tariff) charge roughly 7.3p (inc VAT) for each of the first 223 KWh per month and roughly 3.4p per Kwh thereafter.
So, averaging out that onto a daily basis, there are about 7KWh charged at 7.3p ( = very roughly 50p) and 57KWh charged at 3.4p (which is getting close to £2).
So, as a VERY rough (but not entirely unscientific) estimate, a figure of £2.50 per day might suffice.
Chris