What can I do to make sure plaice filet doesn't get dry?
Firstly consider a lower temprature but do start with a very hot pan, and turn down the heat and make sure you baste your fish correctly; cook skin side for a longer time, untill you have the desired colour. Add your butter and let it foam, and only then turn your fish. Once you have turned your fish you can take your pan off the heat. Start basting with foamy butter. For plaice, It should be done in under 1 minute. If your not serving straight away remove from pan for hot holding. 
If you keep to do this method, its hard to end up with a dry fish.