Hi Mike,
I too bought my ply from B & Q, and the sheets were also slightly warped.
It's the way that they store it, on two small 'stillages' just in from either end of the boards.
I had a similar problem with the base of the TL, and it took me a while to figure it out.
I eventually made a steel frame from 1" x 1" tube, which I was going to use as a chassis, and screwed the base plate to this until assembly was completed. This sorted the problem, and held everything square and flat.
I then found that the tub was rigid, so did away with the heavy frame-work and continued the build as per the manual.
I think it's down to how the timber is stored, even after it's been cut, but not everyone has the space to store flat, so it's stood on end until needed. As most of these projects are built over the dark months, the air is damp, so the timber is prone to warping. Even 'WR' MDF will suffer swelling in these conditions.
You might have better luck with the timber now, as the weather is picking up, but don't give up, you'll get there in the end.
Best Regards,
Mark.