[Mike]

I think you're close to the money, the 21st anniversary logo from 1989 is clearly the inspiration or offspring of the wider rebrand, the differences being a 'Thames' wordmark vs a stylised river Thames.

I'd suggest there was a wider rebrand about to occur and the ITV corporate style came out, which they used for a bit - they probably did double down once they'd found out they'd lost their franchise but it wasn't a case of getting on screen elements and music redone - different tracks probably used to add variety seeing as their only symbol was the same ident each time.