I spun the roadster on spilt diesel on a wet road and bent the chassis, ........... the estimated cost of repair was £33k plus £8k from the Highways agency to repair the crash barriers.
Why would the Highways Agency seek to collect money from you? The accident wasn't your fault - that seems outrageous.
They will seek every opportunity to collect money.
The owner of the vehicle that damages Highways Agency or Council property is deemed liable...and the insurance company will pay.
If, in this case, the source of the diesel spill could be traced then they would go after that person or business.