I think £5 per day is a good starting point. When I was 17 I earned £35 a week on a training scheme and paid my parents £7 a week. They saved it for driving lessons, but it taught me that I had to pay my way. When I was earning £500 a month, they charged me £50 a month. They also saved this and it paid for my wedding (to my surprise).
I think anyone who is over 18 should pay their own way to whatever point their parents decide is necessary. It is great if some people want to and can afford to, subsidise their grown up children, but not everybody can afford to do that.
DC will have to get a Saturday job as soon as they are old enough, in order to pay for mobile phone or whatever.
and they are not paying towards the cost of the house itself, but paying towards the facilities that they use, food, water, electric, broadband, tv etc same as they would if they rented a room in a shared house.
It's another situation that is each to their own isn't it? Some can afford to subsidise, others can't, some can but want their adult children to learn the value of money and paying their own way. Everyone is different.