Good Luck Toby. 
Keep as calm as you can, take a breath before answering anything to give yourself time to get your thoughts together.
Don't force eye contact if it makes you uncomfortable, just be yourself.
Remember, if you don't understand something they say, tell them there and then. Don't get into the situation where you're just trying to fill a space with words, say things like
"I'm sorry, I don't understand the question, please could you clarify"
"I don't understand, Do you mean .." .followed by what you think they mean.
Give yourself time to answer.
Our Judge was fabulous. All the panel know how DWP send cases to Tribunal that they (DWP) should not do because it's so clear-cut the person asking for PIP needs it.
Please let us know how you get on, I'll be thinking of you 