This story has a happy ending.
On Dave's suggestion I took it to a certain exhaust specialist next door to Jaycar in Thomastown. They replaced the dodgy joint with flanges and put in new custom-bent pipes downstream of the extractors, plus a new muffler (the old one was beginning to deteriorate and it wasn't optimised to match the extractors). They did a great job. It's louder than the old set-up. For better or worse it sounds a bit more like a V8 now.
What's really pleasing is the power boost and the smoother responsiveness of the engine from the combination of the off-the-shelf Redback extractors and the new exhaust system. The extractors by themselves were a mild improvement, but together with the matched exhaust it's a quite substantial improvement. I took it for a test drive with my brother, who has a late-model Defender and drove my Series 3 before and after the changes. He was impressed: it climbed a steep hill in high ratio second that it could only manage before in high ratio first, and that was without an adult passenger, a child and a dog on board.
I told the exhaust shop I wanted the sweet spot between too restrictive (too much 'bad' back pressure) and too free (not enough 'good' back pressure, if I understand it correctly). They said 2.25 inch diameter tubes were optimal: any bigger might kill low-end torque. They also advised against a hot dog style muffler (too free) and recommended a baffled muffler, which is freer than the one it replaced yet still provides the necessary back pressure. I went with their advice.
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As for the more grunty engine note, especially on acceleration, I don't mind it but as a family touring vehicle I'll have to do a bit of sound deadening down the track.
