I was at that ren faire gig in maryland in 2007. I did an interview with them and wrote an article on the group. Here's a bit of the background part:

"The first thing they did was set the record straight. Mediæval Bæbes are often accused of being a flash-over-substance act created by record company executives with an eye toward making lots of money, along the lines of Boyzone or the Bæbes' original labelmates, the Spice Girls. But Mediæval Bæbes was a group born of artistic inspiration, not marketing acumen, and it was formed by [bandleader Katharine] Blake, not by a record company or management team. Blake was classically trained at the Purcell School, Britain's oldest specialist school for gifted young musicians, and was a successful rock bandleader and singer before creating the Bæbes in 1996. Furthermore, the individual band members do a lot of textual and musical work on each song, adapting the lyrics and often writing and arranging the music themselves. Blake points out that it's a misconception that they perform mostly medieval music. In fact, the bulk of what they perform is medieval poetry of various sorts, set to entirely original music written by band members.

The seeds of Mediæval Bæbes were sown in two projects Blake had been involved in long before the band's formation. One was a band called Synfonie, which performed music of Hildegard von Bingen. The other was Miranda Sex Garden, a goth-rock outfit that began as a trio of women singing Elizabethan madrigals. According to rock 'n' roll legend, the members of Miranda Sex Garden were out busking in Portobello Road, singing madrigals, when they were spotted by Barry Adamson of the Bad Seeds. Adamson invited them to perform on a soundtrack he was composing for the 1991 film Delusion. On the strength of that performance, they got a record deal, and recorded the first Miranda Sex Garden album, Madra [Mute Records]. This work strongly foreshadows Mediæval Bæbes, consisting simply of Blake, Kelly McCusker, and Jocelyn West singing Elizabethan art songs in unaccompanied harmony. Some prominent critics loved it, with the Evening Standard commenting, "the tabernacle in which we sit is transformed by sheer dint of harmony and poise into another place, where the sky lies like a canopy of blue velvet and the brightest stars are Miranda Sex Garden."

They aren't all great musicians or singers, it must be said. But several of the singers are quite good, and the accompanists are very good as well.

By the way, they are not trying at all for authentic performance practice, just what they think of as interesting medieval sounds. Hence, they use cittern and lute hurdy-gurdy and old viols of various sorts, and recorders and drums. On most gigs they have 2-3 accompanists, one or two men and one woman. That one youtube video with the violin played by Katharine Blake was an anomaly in that respect.

I'd say it's worth the trip!