Hi again John
£45 is a pretty good price for what you have there. The antenna you mention (Watson 770B) is one that I have too, and I must admit that for the price they cannot be beaten.
When I came to set my antenna up on the car the SWR was low on 2m but about 2.5:1 or worse on 70cm. After checking the system with an antenna analyser I noticed that either side of 70cm the SWR was low, so I reasoned that I was seeing the result of an unwanted phase shift on the coaxial feeder of the antenna mount I was using.
After shortening the cable by about three inches I had a nice 1:1 SWR on 70cm and 2m was still 1:1 too.
I've also experimented with a mini-mag antenna and found that although very small the antenna also performs well.
It's good to see that you adapted a bracket, adaption and improvisation are the marks of 'real' Radio Amateurs! Much better than just buying 'off the shelf' items, and way more satisfying!
73 for now, Mark.