I decided that I would make a new one myself using the left one as a mirror image template. I first of all tried this in pine, which was easy and then went on to ash, which proved to be much more difficult (with the tools at my disposal). One of the hardest bits was to get the joint exactly right so that the top of wheel arch wood sat in the correct place. This demanded sub-mm precision on the joint between the two. What you see in the pictures are a result of weeks of cutting, sanding trial fitting, careful measurement and repeating the process. As far as I can measure, the tub is now symmetrical down the centre line to within 1-2mm.
I've also adopted the strengthening bracket modification as recommended by MS in TC Forever More. You can see from the pictures that this is invisible once the correct reinforcing triangular bit is in place. However, I decided not to go for his side screen box modification as it seems like a step too far from the original for my liking.
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Now on to stage 3 ...