In article ,
Peter writes:
Mr sheen oven cleaner.

Thanks, I don't know why I didn't try this first but I squirted it
with Mr Muscle oven cleaner and it dissolved it in under an hour.

I wonder what the active ingredient in that aerosol is?

Probably Sodium Hydroxide -- it should say on the container.
In that case, I suspect your sugar syrup had also contained
some fats, as Sodium Hydroxide is good at breaking down burnt
(polymerised) fat.

--
Andrew Gabriel