This one came up against a new opponent last week.

I had normally played that my floating General could attach to an UG for movement purposes, thereby not requiring a card to move. Obviously, he would count out of command if he tried to prompt that (or any other UG).

However, my new opponent (who was very pleasant to play by the way), believed that a floating General had to remain separate from all UGs and could only move by playing a card.

While it is not explicitly covered in the rules, I believe there is enough implicit information to indicate that my interpretation is correct. Can I have a ruling please?

You are correct.

Page 79 section 8 "General's Movement" says that a general can move with an UG they are with "whoever prompts that move".

Thank you

Yes they can be floating and move around with an UG prompted by others. Indeed classic Alexandrian behaviour to do so and then take control of that one TuG of companions.