What do you do if you're a megabucks musical? You carry at least two understudies for every major role, that's what. But if you're the tiny Mikron Theatre Company just four actors, touring on a canal narrowboat spare cast members are not an option. "Avoid that boat out there. There's a plague' sign on the door," warned actor Daniel Wexler. The boat in question was Mikron's narrowboat Tyseley, this year celebrating her 70th birthday. On board was actor Ruth Cataroche, who definitely wasn't celebrating anything. Just a couple of hours before the show at the Bluebell Tearoom, Marsworth, near Aylesbury, she went down with tonsillitis. Mikron reckon that this was only the third time an actor had missed a show through illness in the company's 35-year history, a very remarkable record.

Ruth was due to play the title role in Mrs Brunel, Mikron's new show about the little-known wife of legendary engineer Isambard Kingdom Brunel. So what to do? "We'll sing the songs, and make stuff up," Daniel told the assembled audience in disarming tones. Inevitably, we didn't get to hear much about Mary Brunel, but we still learnt quite a bit about her husband not least that he was hardly favoured by the posh aristocracy: "His atmospheric railway is a failure," announced one titled gentleman dismissively, "Stopping the train was a bit of a problem".

Perhaps that gentleman didn't know that Brunel could be a man of outstanding personal bravery: the song Man and Boy, set to a suitably swaying melody, tells the story of how Brunel himself clambered out on to the unfinished Clifton Suspension Bridge, to free a jammed construction cradle.

Mikron's other show this year is Carrying On, which throws the spotlight on W.J.Yarwood & Sons, a famous shipyard based at Northwich, on the River Weaver an appropriate choice this, as Yarwoods built Tyseley 70 years ago. In the absence of Mrs Brunel, we heard some of the musical numbers from this show too: I particularly enjoyed "Sing a song of salt, me boys/Pepper it with rhymes/Chuck it over your shoulder/and turn round 20 times", sung in a jaunty George Formby-type arrangement featuring three wildly-strummed banjos.

Cast members Emilia Brodie, Robert Took, and Daniel Wexler carried all this off with considerable aplomb, and much infectious energy, thoroughly whetting the appetite for the two complete shows themselves.

Mikron perform Carrying On at the Ferryman Inn, Bablock Hythe, tonight, and at the Queens Head, Eynsham (Sunday), the Boat, Thrupp (Monday), the Plough, Long Wittenham (Thursday), and the Coal Wharf, Cropredy (July 16). Mrs Brunel is at the Plough, Wolvercote Green (tomorrow), the Boat, Thrupp (Tuesday), and the Coal Wharf, Cropredy (July 17).