Now is a good time to have a builder or engineer who is licensed or insured to check if your deck is safe underneath.

The balustrade should be a safe as possible, and constructed strongly to last for years to come.

"The existing railing needs be brought up 100mm in height to meet building regulations"

Not going to happen with any sort of "extension". Replace it is about your only option.

There doesn't appear to be any posts, just balusters That railing needs a rework.

Looks like you're right. Time for posts.

There's definitely posts, just doesn't show well from this angle

Don't compromise on safety.

gafrine: Your method will work.