Hi Craig,
I just put a smear of copper grease on them and they held in place no problems.
I am at a loss to suggest any reason (other than below) why yours has become dislodged but it could well be that it stays where it is now, just keep a regular check on it until you visit Henry's again.
Thanks Richard, after reading up including watching You Tube vids my understanding these anti-squeal vibration plates are either fixed in place with lugs, or glued to the back of the pad. Glue can break down with heat and the plates work loose. I am not heavy on the brakes so I can rule that out.
I spoke with Tony at Williams this morning, he said they see many Morgan's with these plates working loose. They remove them altogether and apply Anti squeal lubricant paste. My next trip will be to a local garage and probably have them removed.
Thanks also to RichardV6, TBM, DW for coming back to me.