The simple answer is that we don't know. There are all sorts of possibilities from giving background information thru passing on privileged information he got as an MP or by talking to other MPs. Unlikely to have been an actual spy, I think. Too openly left wing. https://twitter.com/lukeking34/status/965976714202492928 …
I'm sure they wanted to know absolutely everything that was going on inside the Labour Party, from multiple sources where possible.