This weekend will be a time of bittersweet celebration for The Song Company as they perform two concerts. On Saturday, at St. Christopher's Cathedral, they will repeat a performance of what artistic director Roland Peelman describes as "Bach's universal compositional artistic statement," his Mass in B minor, which the group, with orchestral accompaniment by the Wallfisch Band, last performed at Albert Hall at the Canberra International Music Festival in 2012. "It's not often heard," Peelman says, "and it was an extraordinary success."

On Sunday they will farewell Clive Birch, a member of The Song Company for 25 years, at an evening concert at Wesley Music Centre. "I've been exclusively with The Song Company for all that time," Birch says. "I feel a little bit of sadness but life goes on. After 25 years it's time to let the next generation have a go I think."

Peelman is delighted to be performing the Bach work at St. Christopher's. "It's a beautiful place to play, with a great acoustic for singers," he says. This time there will be 10 singers. "It was always our intention to have 10 singers in 2012 but we just couldn't find the right 10th person, so we did it with 9." Peelman explains. "Bach wrote most of the pieces in this work for 5 voices and the majority of the choruses were written for 5 parts but the Sanctus is written for 6 parts and then the Hosanna is for 8 parts – a double chorus. So strictly speaking you can't do it with less than 8 singers to have all the lines covered." Peelman further explains that if you have a full complement of 10 singers, then, at certain moments, when the orchestra is very loud, you can have two singers per part. "That is the rationale behind this."

For this performance the orchestra will be Ironwood, a group of 18th specialists, including Daniel Yeadon and Neal Peres da Costa, who play historical instruments: strings, oboes, trumpets flutes, bassoons, "and even a hunting horn. You need that for one number only," Peelman says.

Bach only finished the B minor Mass a little time before he died; he never heard it performed. "It goes far beyond the small parochial environment where he worked; far beyond the Lutheran traditions in Leipzig, far beyond the cantatas that he had spent his lifetime writing," Peelman says. "It's like a total view of the universe from his point of view." Joining the core Song Company singers will be Tobias Cole to sing the alto part and tenor Andrew Goodwin, as well as Alex Oomens, a young soprano new to the ensemble, Sarah Bishop and Mina Kanarides.

Clive Birch nominates participating in the B minor Mass as one of the highlights of his career with The Song Company. "It's extraordinary to be able to sing the whole of the work as a soloist," he says. "The whole detail of the choruses comes out. You can hear every single element of the music; the whole contrapuntal element is there. It's a big thing for us to sing and after a performance you're absolutely exhausted."

Birch is a versatile performer. He allegedly made his debut at the age of five singing Jesus wants me for a sunbeam at Sunday school and he went on to join the D'Oyly Carte Opera Company in England, doing 8 performances a week for 48 weeks of the year. It went on for two and a half years."I got addicted to touring," he says. "I did a lot of comedy roles: Ko-Ko and the Major General and John Wellington Wells – that was one of my favourite characters - but quite honestly I wouldn't mind if Ii never heard another G&S again."

Other highlights of his career have been a Song Company tour to Europe in 1997 when there was a standing ovation after the very first performance and a trip to Groningen in the Netherlands when the group performed Martin Wesley-Smith's Quito. "It's an amazing multi-media piece based around mental health and East Timor," Birch says. "We finished the piece to complete silence and then there was this enormous 'whoosh' and the whole audience got up and just exploded. It was one of the most moving experiences of my life."

And what's going to happen at his farewell concert? "It'll be a big surprise, I think," he says. "Just a farewell and a big thank you to our Canberra audience because we've got some particularly wonderful fans in Canberra, very staunch; they come to every concert. After I retire if the phone doesn't ring I won't mind. But I want to do a lot of walking – maybe the Camino in Spain. I love my walking."