The runway available to backtrack on 10 at Leicester is around 200 metres (according to Google maps). So why only backtrack for 20 metres before turning around? As already pointed out, seems hardly worth it. If you were genuinely planning to backtrack the entire length of the runway, the fact that the Pitts nipped in and off by the time you'd made 20 metres suggests to me that he was well in time to allow you to have made a full backtrack of the 200 metres available.
To note - biplane / vintage radios are often rubbish. The chances are that the call entering the runway was made, but won't necessarily have been very clear making it easy to miss whilst talking to your student.
If you didn't really need to backtrack, since this magical aircraft will outperform a Pitts, then it could be argued that you were being rude by occupying the runway for an excessive and un-necessary amount of time.
By the way, I'd rather have a Pitts take off in front of me than to take off and worry about having one climbing up my chuff.