I've done a bit of research on this and if you exclude mooring fees and assuming you own the boat outright then £1k a month is in the ball park. That includes fuel, food, internet, licences, insurance, maintenance fund etc (assuming its a fairly modern boat in good condition to start with).
If you add in a marina mooring (depending on where abouts in the country that is) you can add roughly £3k a year (around the midlands) or much more around London.
I'd be interested in other peoples actual figures if they differ vastly from this!