Thanks for all the advice. Given that we're not 100% sure of the nature of the reference our DD's current school will provide (it's a "through" independent school - i.e. junior and senior - so it's not really going to be in their interests to "market" her to potential competitor schools) we've decided to include a brief (mostly bullet-point) covering letter with the application to act as a "highlight reel" for her accomplishments both within school and without.
If the application form leads to other requests for more detailed information, that that's fine - but we don't want to risk a potential new school not being aware of awards etc. to which we'd like to draw their attention at the outset.
That is a good idea if the school does not have a strong motivation to market the child's case.