My husband, ~23-month son and I stayed at Helen and Huw's place while we were in Bristol visiting some friends. It was a great find. They have essentially turned their basement into a self-contained flat with its own entrance. The place is very spacious and had everything we needed. They even provided a travel cot and some baby toys and books. There are some stairs inside (which presumably once led up to the main house) and also some quite steep steps down to the flat entrance, but other than that it was very baby friendly.
The location, very near Gloucester Road, was great. This is a trendy area with lots of cafes, restaurants and pubs, and the flat itself is on a leafy and quiet road that felt very safe. Not sure how easy it is to get into central Bristol as we didn't venture in that direction.