>I would say at least �1000, if you can.
When I first saw this figure I nodded in agreement. Then I began to think about it.
�1000 for 3 weeks (20 days), that is �50 a day.
2 adults, 1 teenager, and 2 children (lets call that 4 "people"), so that is just over �10 a day each!
I dont think you are going to get by on �50 a day for 4 people.
Even if you already have theme park tickets there is still the cost of eating and drinking, petrol (if you have a car), parking (if you have a car - all the theme parks charge you to park), taxi or bus fares (if you dont have a car), snacks, ice creams, presents and gifts, any shopping you do (clothes etc), postcards and stamps and all sorts of other things.
To be honest, you can spend �50 in one long day in a theme park on food, drink, snacks etc.
My guess is you will probably spend between �1500 to �2000 during the 3 weeks you are there.