Thanks everyone!

I just did an experiment.

One hotplate I used bicarb and vinegar. The other I used spray on oven cleaner (Easy Oven or something).

I left for 10 minutes or so then scrubbed with steel wool pads.

The winner?? The bicarb and vinegar. They are not entirely clean as I ran out of muscle power but they are much, much better than they were. I will do it again tomorrow.

Thanks