Somehow I'm not sure either Sam or I anticipated our collaboration would lead to the two of us driving around The Dings, looking for a store called "Bus Parts." But lo, here we are, knocking on Bus Part's door and asking a slightly bemused Brummie gentleman whether they sell anything in flexible rubber that might act as a form of bellows for a custom-built harmonium unit.

Our friend pops an unlikely moulded protrusion onto the counter. It's shaped like a bullet, and the size of a football. We feel it up. "Too hard." "Yep, that's not really gonna give." "Yes," Mr Bus Parts counters, "It is sort of designed to support weights of up to 7 tonnes." He helpfully suggests that air conditioning suppliers might have rubber offcuts to suit our needs. "Either that," he says, "Or pop down to Lawrence Hill bus depot. They've got rubber sheeting hanging from the ceilings. Most of it unusable."

At the recycling centre they've clamped down on out-of-town vehicles, so apparently, if we try to offload a woodworm infested harmonium from Sam's out-of-town van, we'll be torpedoed on sight as a matter of principle. All this to add to our woes. "We've had get tough!" says the guy at the crusher, in a jolly sort of lilt, "Everyone used to just dump stuff. Took the piss. Commercial stuff. We all did it! I'm from Hengrove. Everyone's a criminal there! We've all done time!" He gives us a form to fill in. The form is full of official warnings and remonstrations. Dumping your shit? It just got real.

Later on we're in B&Q, and perturbing the floor staff with oblique requests. "This is what a lot of my life is like now," says Sam. "Walking around hardware stores, trying to find things that are a bit like other things." We spend a while frowning over screw fittings and wondering whether you could make bellows out of canvas. At one point Sam looks at some wellington boots, wistfully. "Something just like this would be perfect," he says, "But preferably not in the form of a boot."

Yes. That's right. This week we're making it up as we go along, and for the following reasons:

1. The weather. We need to dismantle my woodworm-riddled harmonium to get at the brass reeds within, but it's pissing it down, and for fear of contamination we can't bring the instrument into my house (wooden floors) or the Watershed (wooden everything.)

2. We still need a solution to our power problem. An MP3 contained within our harmonium unit has to spring to life after the instrument is played for a minute or so. But how to do this? Dynamo and battery power are being investigated. So we figure that to address some of these critical issues we need to start building a basic mock-up of our instrument... out of MDF, gaffer tape, silicone sealant, and (as it transpires) B&Q's second finest kind of flexible ventilation tubing.

3. The best kind of head-scratcher: the joys and possibilities thrown up by reaction to last week's Work In Progress session at the studio. We're rethinking the 'provenance' of our harmonium units, where precisely we might leave them to be discovered, how simply we could present them, how free and uncomplicated we could make their operation.

But by the end of the week we're back on course. We've managed to fillet the harmonium (in a few scant hours of sunshine) and build a prototype housing, reed-less but successfully tested by means of a plastic toy trumpet jammed over the sounding hole (a plastic toy trumpet that I simply KNEW was hanging around the house for some god-given reason, and now, at last, its time has come.)

Our mockup is an odd looking fish that I decide could pass for an East German-issue wooden lung. Its intake valve is a scrap of theatre lighting gel (colour: light blue.) It has a slightly different personality depending on whether you operate it bellows-up or side-on. And one simple but obvious outcome of Sam's lighting quick work on the prototype is that we can now imagine what it might be like to stumble upon it somewhere, what might be written or displayed on the surface, what discovering its sonic functions might entail.

The harmonium itself is recorded in the garden – with the roar of the M32 forming a shifting accompaniment to its chords – one last time before we disassemble the sad old thing. The changing weather has stuck its sounding board onto one basic setting. Some notes are suffering, and fizz like an angry bee in a bottle. I wonder if these chords I'm playing will ever get the chance to form again; half of the joy of this project is that even if they do, I might never know it.