Hi Neil,
I had a job today excactly the same. Pure white carpet with tea stains. Tried all the usual tricks, tea & coffee remover, browning prescripion. But didn't totally remove all tea. I was disappionted as i generly have good results.
On speaking to prochem they recommened oxibrite. I didn't have any so don't know for sure if it will work.
As i made no promises to the customer we were both happy and i got another job from them. Not a white carpet this time thank god, i don't want to see another one again.
Rob.