Non-traditional concert spaces can present all sorts of problems, and the high-ceilinged, alcove-ridden entrance hall of the Ashmolean doesn't look very acoustically hopeful. What you'd need to avoid an a cappella choral concert like this devolving into soup was a group of very talented singers who were completely used to the space they were working in.

Luckily, that was what we had. John Lubbock's choral extension to the Orchestra of St John's, Ashmolean Voices, specifically assembled for the series of prom concerts staged here. Clear as a bell throughout and exploiting extraordinary dynamic range and control meant we didn't miss a note, whether we were hearing dissonant modern arrangements, lapidary Tudor anthems or the knotty harmonies of Bruckner (a sinuous 'Virga Jesse').

The undoubted highlight of the concert was the two versions of the German carol 'Es ist ein Ros entsprungen'. We were prepared for Swedish composer Jan Sandstrom's extraordinary slow-motion version of this 17th century German carol by hearing it in its original form at a more stately pace than usual. This was followed by Sandstrom's gloss – the tune itself now trapped in a slow-flowing amber of shifting, hummed drones that maintained uneasy dissonances far longer than the ear is used to.

Clearly inspired by Sandstrom, Lubbock's own arrangement of 'Away in a Manger' which closed the first half was a little more harmonically traditional, but used a similar effect well.

The second half was opened by an short impromptu art lecture that was occasionally interesting but seemed a little under-cooked and took a rather cynical left turn into an attempt to promote an upcoming concert.

Musically, things became a bit more of a slog – we were subjected to a lot of rather solemn, glutinous fare. The choir remained crisp and disciplined throughout – although 'In Dulci Jubilo', and 'The Shepherd's Cradle Song' by MacPherson, provided rare moments of murk that tempted the attention to wander. The problem was more in the programming, which could have done with more than the two brief moments of levity (a rousing rendition of 'Torches' being one) than it scheduled.

The group must have known this, however, because they encored with a maverick, conductor-less rendition of 'We Wish you a Merry Christmas' that left everything pretty much forgiven and showcased exactly how much each member enjoyed being part of this highly talented outfit.