Originally Posted by
myles
I don't know how to stop it, but the dash was redesigned at some point. Both are very similar in appearance. The early dash had a long slim opening on each side for the demister vent. The later one has some bridging between the front and back edges of the vent, though they are recessed a few mm below the top of the dash, still visible if you look in the vent. The later ones are much more resistant to warping out of shape.
I removed my dash pad to examine the problem and when the dash is removed it sits almost perfectly flat. The ducting that is attached to the underside of the dash is actually what pushes the dash out of shape. If you put some effort into positioning the dash and ducting more accurately on the heater box and mounting points it can be considerably improved.