The surface underneath has darkened because of excess heat and it is impossible to change the colour back again naturally.
Your choices are either to sand the whole thing off to remove enough of the wood surface that is damaged then revarnish, or disguise the mark by colouring it on top of the mark.
Hi Buildersmate. It's a friend who's done this and she said that the table has gone lighter, as if it's been bleached. Would you still suggest the same solution? Thanks. :)
We did this once as students and rubbed some furniture wax into it which seemed to at least disguise (darken down) the mark if not remove it completely