It worked out fine in the end in case anyone's interested. I rang T before I left and she told me to just knock when I get there and we'll start the session from my arrival time. I reminded her I'd had to book another cab at the usual ending time but she said that was okay. As it happened the cab was slightly late collecting me and then the roads were bad due to all the rain and flooding so by the time I arrived there it was only about one minute early. T agreed it would've been unreasonable for me to wait outside. The return cab was a few minutes late collecting me from T's house but she said we could sit and wait in the lounge. By the time our session ended and I'd gone downstairs to sit in the lounge the cab turned up. So all's well that end's well. Not something I'd like to do everytime but it was nice that T was willing to accomodate me.