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Clearly the council know the sign has been removed and the hole repaired. I would say that the council are liable as the absence of a bollard has caused the incident just as they are liable for a pothole with council white paint around it. Once they know about a defect they have a duty of care as far as I can see.
Have a look at https://www.blclaims.co.uk/latest-news/news/2013/01/18/highways_and_potholes/
also search google for - Highway Risks and Liability ClaimsTake photos of any other islands in the same street with the bollard/s in place to prove your point when required.
Turns out there was originally a neon yellow plastic bollard thing on the island, which was knocked off and the hole just filled, with it not being replaced. The other islands along the road do still have them, so more promising that Joel can get someone to pay for his teeth.