Could anyone suggest a site where i may find some information about this?

I'm sorry you haven't had a reply.I can only give you my second hand advice on this plant.(Next doors came into our garden uninvited ).I believe they don't get new leaves on old wood so I wouldn't prune back too hard.
If you cut down the tree as our neighbour did you will get lots of suckers growing,it may do the same if the original plant is dying.
There are some comments on this site from owners,try entering 'rhus typhina' in the custom search box to read care and growing tips.You could also 'google' the same for wider advice.
I am not sure what colour the inside of the stems should be,but there is a wilt that they can get,you may find more about this on line.
Good luck I hope you find a solution :)

Thanks for that, I'll have a search about.
Frankly if it's not going to grow leaves till next year it's going anyway. I've only kept it because it provides lots of shade for the kids.
There are suckers popping up everywhere, so if I do cut it down I'll treat the roots with something containing Ammonium Sulphamate. I used this stuff on some stubborn Ivy a few years ago and it never came back. I'm told it can be used on tree stumps to prevent regrowth. I'm hoping that it will remove all the suckers as well, as it's systemic.