I hadn't seen this for decades until just now, remembering it as a not very good film with some very good moments. The big trouble for William fans like me is we've all got our own conceptions of William and his world making it impossible for film-makers to please even most of us. This is an episodic film taking bits out of many of the books with some wildly inaccurate characterisation, but with something that should override all complaints: It was filmed pre-War and therefore couldn't help but be faithful to the original atmosphere. It doesn't matter whether the Brown's are living in a mansion - the family and class relationships are all there. Thankfully a lot of the jokes are too!
The main thread is William and the Outlaws are obsessed with catching someone they perceive to be Dynamite Dan and his evil cohort, who rob Mrs Bott of a pearl necklace (Mr. Bott has no part, being deceased). Roddy McDowell played Ginger, Aubrey Mather the baddie - who wasn't very long following him to America as an evacuee. Basil Radford as Uncle Percy was a bit wasted, and Fred Emney was not my idea of William's father - at one point at the breakfast table he calls Mrs Brown "an idiotic little woman". I've got a record his Dad made in 1912, now he was funny. Dicky Lupino playing William was OK if maybe too soft-looking and a stone chunkier than the ideal; Douglas was Scottish! A story that was banned from being published by Macmillan in the '90's, William and the Nasties is touched on, and a baby show where you want it to lead to a conclusion with the enormous title "That was what she said before she saw the baby!" but the opportunity was missed.
A lot was packed into this however, so it's a shame to nit-pick on comparisons with the books - it's a fair 1930's b&w British family comedy on its own merits, worth a look in. How do you tell when someone has shaved off a false beard is one of the many profound questions thrown out from William we'll never know the answer to.