At this rate, Harry, I've a good mind to stick some oxallic acid crystals in an envelope and post it to you...
Always best to wait until all answers are in, before committing. It is though, looking like a burn/heat stain. Will oxalic tackle that?
I'm planning a trip the Home Base today, for the Bar Keepers - I have a plastic composite thingummy sink, in the kitchen which stains very easily and a stainless one in the utility, so it might work for the prior.