Just a quick update - I read an article saying that FD may be a reaction to a bacteria called Staphylococcus Aureus.
My FD started around the time that I was visiting my dear father daily in hospital over a two month period. I know that there was MRSA in the hospital, as well as other bugs. Poor Dad caught it twice and it was a contributory factor in him not recovering.
I hadn't put this connection together before, but it is perhaps a possibility.