The official meaning of code TT is "Autumn-attribution Neutral Zone delays
Adhesion delays are normally attributed to Network Rail (QH/QI/QJ), and the neutral zones are defined areas where such delays, up to an agreed limit, taken out of the attribution process (though where they go isn't explained in the DAPR).
Autumn 'TT' delays are split 50/50 between Network Rail and the train operator.