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Fab, thank you both for your thoughts. I was 99% sure I was going to do it, just needed someone else to agree with me

I would definitely not allow search engines to index those type of results pages. To be fair, they're unlikely to come across them as a bot wouldn't typically fill in a search box to search, but ey might follow a link from somewhere else. For products, I would definitely want to be using category (or similar) pages to define what the search engines saw.