There are two relays in the main group that control main beam, dip/main changeover and main flash but both are wired before the main beam fuse so couldn't cause it to blow.
The fault is a likely a cable short after fuse 16 which feeds a blue/white wire (UW74) to power headlamp main beam. There is a three way split in this wire at the inboard end to feed each headlamp (UW75/76) with third wire (UW77) feeding back to give high beam warning on the panel.
Hopefully this gives some guidance as to where to look for a short.
