Thanks for your responses everyone. It's so helpful to see how other families manage it.
DH & I sat down with DD to discuss screen time rules. She had written down a couple of sensible ones herself and we added to them letting her know our reasons.
The tablet itself has the feature which makes it possible to set a daily time limit. DD came up with 1.5 hours herself where I had thought maybe 2 hours but in the end kept that to myself! It at least gives us some wiggle room for later on depending on how we get on.
The time doesn't include homework using the school Apps and if she has friends over she may get more time as she has to 'share' her time. The time limit does include time spent on other games consoles/devices.
Things such as getting ready for school, eating, bedtime, homework, chores have priority over tablet/screen time.
Screen time ends one hour before bedtime.
Tablet is not allowed upstairs. Although that could be contentious as much younger DC has been allowed to watch nursery rhymes on my phone/iPad because they wake up ridiculously early (only after 6am) This allows me and DH to get more sleep/shower in peace.
Tablet must be put away safely when not using (one of DD's rules). It's her responsibility.
I do think that DD needs to have restrictions on the time she is allowed to play. She would struggle to put it down otherwise and she can get quite stroppy after spending time on consoles/tablets. However, I don't want screen time to be used as a behaviour management tool. I think that if we have set rules she knows where she stands.