Hi All,
Many thanks for all the feedback and suggestions. As a new member to this forum it has been great to receive such support. Just to confirm, the connectors shown are not fuse holders. The white connectors are female (RED/BLUE and RED GREEN wires) and the black connector (RED Wire) is male. This is the reason I was originally concerned that the RED may have become disconnected from either the RED/BLUE or RED/GREEN. It looks from the discussions like this is somewhat unique to very late model 1500s! As mentioned in the original post I did consult the Haynes wiring diagram but as a Mechanical Engineer I struggle with anything that cannot be fixed by a hammer or a spanner! For now I am going to go with the optional Rear Fog Light hypothesis but will make sure the stray BLACK wire is correctly earthed. The "spare wires" all have connectors on the ends so it should be safe to tuck them away again behind the passenger glove box.
For anyone who is interested, below is my Spit. Bought as a 21st Birthday present to myself...over 30 years ago 🙂