Can anyone source this quote?

Update: Mystery solved! The quote is a genuine Pratchett one, and it comes from this interview he did with the Science Fiction Book Club in the late 1990s.

The other day, I was having one of my periodic bouts of reading around Fimfiction blogs. I like doing that from time to time, as I sometimes find some interesting stuff that way. Admittedly I also find some very boring posts – but hey, you're reading Louder Yay, so you should be used to that sort of thing.

This time, I was reading a blog by someone who I'll leave unnamed. That's partly because it's not relevant to my question, but also because they'd been involved in a nasty argument in a Fimfiction group and because context suggested they'd been through a very rough time in their personal life. Not things I want to get involved in.

It was a long blog about writing, and part of it was taken up by the author rejecting the existence of writer's block as a real condition. That's not the point I want to discuss here, though. This is the quote in question, which was attributed to Terry Pratchett:

There's no such thing as writer's block. That was invented by people in California who couldn't write.

Now, I've spent a fair amount of time Googling, both standard search and Books, and I've discovered two things. One, this line is quoted in many places. Two, it's quoted by people who don't provide any kind of source for it. This happens a lot on the internet especially – just the other week, a quote went the rounds on Twitter, also attributed to Pterry:

Satire is meant to ridicule power. If you are laughing at people who are hurting, it's not satire, it's bullying.

That one was investigated by a number of people and they could find no evidence that Pratchett had ever said it – yet it will probably now stick in many people's minds as "that cool Terry Pratchett quote about satire" or something. As with so much else, it makes me grateful that way back in middle school we had a timetabled class called "Study Skills" which covered checking sources. (This was in pre-internet days, I should say.)

So, back to the writer's block quote. As I said a minute ago, I'm not actually interested right now in whether people agree with it or not. However, is anyone reading this able to provide an actual source for Pratchett having said it – or, just as good, hard evidence that he didn't? Thanks!