Use an ammeter in series with the mains supply, then add each load on one at a time and note the currents, the final reading will be the total fuse required.

Alternately you could fuse each load separately for better safety, i assume all loads are mains powered and not 12V dc.

 

Thanks a million for the help - I put in a 10amp slow blow fuse, and the new PSU, and its working great.