Moultons are nice, most made to measure and that is where their best worth is with the original purchaser. So ride it.
If you want to keep it, as a future downpayment for a house, keep it indoors. Framesaver appears to be a good product, EXCEPT if the bike has chromed head lugs, there it is the chrome plater who will have defined the life of the frame.
Dave Moulton also made a number of frames prior to coming to the states, I do not think that 216 number includes bikes made there.