I think I read a bit of The Worst Witch years ago ... It didn't capture me, though I have only very few memories. But even if there are similarities, Harry Potter will hardly be a copy of it - Harry Potter captured me instantly, and that was before the big hype started and the books were everpresent (my aunt sent us copies of the first two books after book 3 had been released in the US)
Pratchett has sold 40+ million books worldwide, he hardly has a reason to be jealous of anyone, and I find a single one of his books far more interesting than anything Rowling can write.
Which is a tenth of the amount of books Rowling has sold. Rowling's books are second in international popularity only to the bible bible .. while his works are still, also if they're HUGE, a "side genre" matter - yeah, they're famous, but they aren't the popular culture event of the last 30 years, as Rowling's books are. In every single interview I've seen with him, whenever Harry Potter comes up, he does seem QUITE pissed about the fact that she managed, in a few years, what he did not manage in his lifetime. Which I, coming from someone who has been as sucessful as he has been, find quite pathetic really.
How inspired his work is is really a matter of taste. He's the kind of writer where each line of his makes me want to smack him in his face, so I am a bit biased there
.. I'm sure he's quite inspired, and that his works have a lot in them for a lot of people. But he has to deal with the fact that his works, no matter what one can, as a critic, say about them, have not managed to capture the entire planet the way Rowling has.
As for him not being the first person to accuse Rowling of plagiarism: What do you expect? You can, in the present day and age, not be successful in anything without having shit thrown at you from all corners. I quite noticed it with my brother, who is a professional cyclist - as soon as he started winning races in major categories, rumours started turning up that he was doping. As soon as he won the national time trial championships, even Austrian state TV reported that he was probably doped. I personally find this attitude of instant hatred towards people that have been successful quite disgusting.
It doesn't mean a thing how many people have accused her of plagiarism, especially if they can't decide WHAT she plagiarised from - Lord of the Rings? The Wizard of Earthsea? The Hogfather? The Worst Witch? The Legend of Rah and the Muggles?
If you can't figure out what something is plagiarised from, it isn't.