Well, okay, it's not technically a requirement but BT does suggest that 1458 is the correct value to use for their broadband.
http://eco.btwholesale.com/broadband...x.asp#20030131
As I can't think of any other cause of this, you're right in suggesting that the value be changed. Logic would suggest it needs to be lower, but probably not as low as 1400.
Make the changes on your router web admin as before, then reset the router (having saved changes) and try again.