Having to be reminded once, or maybe even twice, could be regarded as just a mistake, a matter of things getting away on a busy person. Being reminded a dozen times makes it look less like forgetfulness and more like deliberate non-compliance. And those assurances look less like the hurried "yeah, yeah, I'll get round to it", and more like outright lies.
The rules in the Cabinet Manual are clear: Ministers must behave in a way that upholds, and is seen to uphold, the highest ethical and behavioural standards. They must declare their conflicts and have them managed. Wood hasn't just fallen below that standard - he appears to be deliberately flouting it. Its not enough anymore to stand him down from his transport portfolio - he has shown that he simply cannot be trusted. He must be removed as a Minister, and never allowed to sit in Cabinet again.