Hi Bungle,
It is a bit of an unanswerable question as every person budgets differently but things you should consider when you are looking at budgeting for your trip.
Included in the price of your holiday are three breakfasts and one dinner so you'll need to budget for one dinner and a couple of lunches. There are plenty of eateries that suit different budgets, when I went to Bruges I worked on the basis of UK prices and it wasn't far off (although this was a couple of years ago).
There are two optional excursions on this trip, Ghent and Ypres which you can book before you leave the UK and therefore not have to worry about when travelling.
On top of that, you'll just need to think about taking any currency for any souvenirs to bring home.
Major credit card are widely accepted but please inform your provider before travelling to avoid any problems on your trip.
I hope that goes some way to helping you think about how much currency to take with you even if it doesn't give you an exact amount.
Have a wonderful time and please let us know how you get on with your first holiday abroad.
Kindest regards,
Julia