Good link there Wow.
Our experiences at Harwich have never been excellent with the lifts being a significant issue as well as port staff not seeming to be fully cognisant with bikes and ferries in spite of it clearly being a popular route for cyclists.
On top of that I would suggest that you give yourself more than 30 minutes just in case trains are delayed or cancelled. Given the incoming rail-bending temperatures and inevitable likelihood of late running trains let alone pending industrial action, I would err on the side of caution. But then, I am known for being a glass half empty, risk averse type of bear.