Does anyone know anything about gardening?

I wouldn't have the slightest hesitation in replacing your decking, regardless of bylaws. The only way there could be any consequences is if the neighbours object, which as that has been there for a while is unlikely. If you're feeling nervous, talk to the neighbours.

For sheds it's measure from the highest point - i.e. the bits where it's 0cm from the ground, not sure if the same applies for decking.