A bet hinges on this one, the kind with little money and a lot of honour. What is the weirdest follow-up 45 to a top 10 hit? I named Pink Floyds's "Apples and Oranges", the follow-up to the magnificent "See Emily Play", but maybe someone knows an even more extreme example. Although I could be completely wrong, as Roger Waters stated "'Apples and Oranges' was destroyed by the production. It's a fucking good song"

I'm not sure I accept your premise that Apples and Oranges is either extremely weird in comparison to Emily or not a great song. I love it and I think it's pretty catchy too. Now, had they released "Scream Thy Last Scream" you may have had a point, though I love that song perhaps more than any other.

I always found The Who's 'I'm A Boy' to be a pretty radical departure from their pop-art noise attacks prior to it, though I'm not even sure 'Substitute' made the top ten.

I also think The Small Faces follow up to Lazy Sunday, The Universal, looks pretty odd and ramshackle.

Agree with you about the Small Faces, Biff. Although If I recall correctly they hated Lazy Sunday and never wanted it released as a single, so maybe that was the aberration and they wanted to go in a more progressive way.
I wonder if Marriott had have stayed with them whether they would have ended up sounding like Humble Pie?

If you include b-sides, the Crocheted Doughnut Ring have "Happy Castle" on their first 45, and "Nice" on their second, and that's a pretty huge shift from pop to soundscape! Although I'm not so sure the Crocheted Doughnut Ring cracked any top ten. So I guess it doesn't qualify anyway. But it is an interesting follow up.