According to NRE this evening though, this particular incident was a tree on the line between Clapton and St James Street. I had a look on Open Train Times and there was indeed a block on the line in between these two stations in the London Bound direction earlier.I can confirm, there are overgrowing tree beaches just out of Liverpool street that I remember the train hitting and hearing a loud thwack some time ago. They have to cut them every so often, they do grow back.
I agree. "line side vegetation" might be a valid reporting code for internal delay attribution, but plain English is always good when talking to the public. After all, lineside vegetation could be leaves on the line (unless that goes under "poor adhesion").