Just fitted Micro DE's on the 1200, fitted to a relay direct to the battery, have a 15amp fuse fitted which has blown this morning, should I have fitted a higher amp or is 15 about right?
You got the right size, it blew before your wiring went into melt down. You likely have a short somewhere, maybe a loose connector, or break in the wiring contacting ground.
A 15 amp fuse should protect about 180 watts (12v*15a), so if your DE's are about 55 watts each you could use a 10 amp.