St Mary Undercroft, which was rebuilt in the nineteenth century, is still a working chapel within the Houses of Parliament. It was decorated in the nineteenth century by Edward Barry.
During its long history it has been a chapel, a civic court, a storage space and a dining room and then a chapel again in the Victorian New Palace of Westminster. There is a story, although it’s sadly not likely to be true, that Oliver Cromwell used it as a stable for his horses. Today it is not a parish church, but is used by Parliament for regular church services and concerts.