Hi Rob,I had a similar problem with metal bits in oil filter ended up as a major strip down of the engine,turned out to be the crankshaft main bearings turning in the crankcase and shaving bits of metal off as it turned.What type of metal is in your filter ,steel,white metal,aluminium?
this may give an indication of where the metal is coming from,when i had the similar problem this site proved invaluable with input from much wiser and experienced enfield owners than me I am sure there will be more advice from this site forthcoming.