Most cheap fares are non-refundable. (You might be able to see the rules more clearly if you replicate the booking at matrix.itasoftware.com)

However if you cancel you should be able to get the £24 govt taxes back (not the fuel surcharge or the fare), less the service fee which is £15 online or £30 on the phone. So it's worth doing online if that's possible, but not offline unless they accept it should be possible online and only charge you £15.