Thanks again. The OCD side of me would love to know how the relay swap cured fuse 16 blowing. Despite pouring over the schematics and much head scratching I'm none the wiser though. Put it down to M3W electrical vageris

A relay has contacts that when used eventually erode and provide less of a solid contact. In doing so the resistance to connection goes up as does the current draw, therefore blowing a fuse.
That's totally understandable Stephen. As I've explained though the particular fuse that blew according to OP, is the main beam fuse 16 which is in an electrical path
after either of the two relays that can supply current directly to main beam filaments.
This is the situation that needs explanation assuming it wasn't a short in this direct cabling (now corrected) that created enough current flow to blow the afore mentioned fuse.