If you were to have a tailwheel rating, then you could book the hour.
Not quite. Apart from the fact that there is no such thing as a 'tailwheel rating', in order to claim any flight time on a single pilot aeroplane, you must be either PIC or P/UT. Since it appears that you have not completed the required differences training for a tailwheel aeroplane, you cannot claim PIC and, unless your friend holds an FI rating and was instructing you at the time, you cannot claim P/UT. Despite what some (including certain elements of the CAA) would have you believe, the privileges of a JAA CRI rating do not extend to differences training. According to Schedule 7 of the ANO,
"A class rating instructor rating (single-pilot aeroplane) entitles the holder to instruct licence holders for the issue of a type or class rating for single-pilot aeroplanes."No more and no less.