Signals and Norton Fitzwarren

I'm just putting together a signal box design in SSI

for Norton Fitzwarren junction just west of Taunton. I have some questions:

1) What is an Adv. Starter for? does it copy another starter or is it operated independently?

2) What are the spring points on the diagram I found it the GWR Historical Layouts book? They look like catch points but they are labelled, whereas the other catch points on the diagram are not.

And also a request,

If anyone has any photos/diagrams/etc of Norton Fitzwarren junction but particularly information relating to the signal box both physically and operationally, I would be happy to pay for any postage or even better if it is on the web or in books that I could get from the library. The diagram I have is abridged and only contains a few signal numbers and does not number the points. I would like to label them correctly both for accuracy and also to help me with my SSI layout.

Thanks

Luke Briner