Yes it's a mutation caused by a virus of some sort. The same thing happened to my Coneflowers. Sorry I can't offer more than this, but it's a start.

Were these seeds you bought this year, or had you saved them yourself from plants last year?

Looks like you got lucky. Looks great!

Thanks for the replies.

It was a packet of seeds I picked up in the supermarket a few months ago. I'm wondering if the seeds from the mutant will produce the same...

Great discoveries were and are being made with the same kind of wondering that you are doing. Experiment by planting them next year.

Sunflowers often do produce multiple heads; the F1 hybrids don't, but in their second year (if the seed is saved) may well do so, that's why I asked if you'd used your own seed. But sometimes, an odd seed in the packet will produce one of these - and yes, if you save the seeds from this one, you may find you get one with even more heads next year. I think the record is over a100, but with that many, they tend to appear up the stem as well.

Thanks so much for all your responses