We went in to see the famous sarcophagus. You can see it from 10 meters away, behind the rope - so I don't really know whether it was wort the visit. Apart from that there's nothing of interest inside.
It's only 50 metres from 5 Wells Square and worth a few minutes to look inside - entry is free / by donation. It's an early Christian three-nave basilica with notable Baroque alter paintings. Note that shoulder covering is needed to enter the church.