captainsmiffy,
No probs. I'm quite curious about this one because I've never seen the Mail app crash myself. It generally just happily does what it says on the tin !
If I were to hazard a guess, there may be something lurking in your Inbox that its failing to parse correctly, which leads me nicely onto...
The other thing you could do is setup a test email account somewhere else (or borrow your son's details for his, if he trusts you sufficiently ! ) .... remove your BT account settings, insert test account settings and see if you can replicate the issue there.
Theoretically the word "could" above should be replaced with "should" ....it shouldn't take too long seeing as you know your way around the iPad email settings screens by now !
Have a good evening.