Hi Alex,
OP says his/her house is worth $650,000, and the new place is now bought for $740,000, so it's worth about $705,000 before stamps. That means total loan is original $200,000 plus $740,000 which is $940,000. Total value of securities is $650,000 + $705,000 = $1,355,000.
$940,000 /...