Wow! Turns out that the Red-backed Shrike at Staines Moor is in fact a Brown Shrike (I was correct in my original thought that it was a 1st winter not a juvenile bird!)
I went back to have a better look today and the number of birders had grown considerably - about 200-300 present.
The bird was showing intermittently so I managed to see some of it's features and got some rather crap pictures.
I did note the long graduated tail and other features that the information on the web described.
I look forward to reading a more in depth identification paper - hopefully in Birding World.