I stayed 3 nights at the St. Mary's Guest House mid-week in May, 2012. It was easy to drive to, especially as we were coming from the North, and was a short walk either on the street or along the river to the Bootham Bar. An added plus was that it was only a street away from my favourite York pub, The Minster. I was there with a friend, so we had a twin on the 1st floor, so only had to negotiate one set of stairs. The room was large enough but not large, however, our bathroom was huge (I suspect it had been a shared bath in earlier days). The week we were there was England's summer (!!) with temperatures in the high 20's, our room was very hot the first night (plus a western exposure) but we asked for lighter bedding in the morning and the duvet was replaced by a sheet and blanket, with added bonus of an electric fan, which was much appreciated. The breakfast was good and plentiful, although neither of us could get into the British custom of the "full English".
Overall, St. Mary's is a nice clean, well located accommodation while in York, We lucked into one of the few off-road parking spots, so I don't know where you might park if you were the last one to arrive.More