Early uses

I've found what may be the wellspring of the bastardised form.

The State of the language by Christopher B. Ricks and Leonard Michaels (1990) quotes from a 1988 introduction to the soap opera Eastenders produced by Lionheart Television (the North American distribution arm of the BBC). It reads:
Angie things Den is nothing but aggro. Den thinks Angie is all mouth and no trousers. Sharon thinks they're both berks. Lofty thinks they're all barmy and wishes they'd just belt up - for once.

Google's book search finds just one earlier use, from the London Theatre Review in 1987, in a reference to a comic hero who is all mouth and no trousers.

The bastardised phrase seems to have its roots in the London-based media of the late 1980s. This was the heyday of the Yorkshire sitcom Last of the Summer Wine which, as has been noted, frequently used 'all mouth and trousers', a phrase which might have previously been unfamiliar in many parts of the capital. As that milieu might exemplify the meaning of the original phrase, it seems little wonder the poor dears got confused.

The Google books thing turns up various earlier uses of 'all mouth and trousers', of course. The earliest I can find is in a short story by LP Hartley, collected in 'Two for the River' (1961):
'I wasn't criticizing you,' I said, 'or them.'
'It's not a bad life. Most men are all mouth and trousers— well, I like the trousers best, if you see what I mean.'
'You mean without the trousers.'
'Yes, I suppose I do.'

Earlier examples of either form would be gratefully noted.

Splodge!

Following the recent Guardian blog debate, I was inspired to have another look for any further information on 'Mouth and Trousers', the 1982 single by punk jesters Spodge which I mentioned a couple of years ago. And behold!

Mind your language

The Guardian's Mind your language blog tackles the biggest question of the age with its latest entry:
Something lacking in the trouser department?
Big debate that has divided the experts: 'all mouth and trousers' v 'all mouth and no trousers'

Production editor David Marsh reports:
A reader has taken the Guardian style guide to task over our preference for the phrase "all mouth and trousers" to "all mouth and no trousers".
He says: "I was just ranting about this in a comment on David Mitchell's latest soap box post, and it occurred to me that I had never written to the style guide itself on the subject of what I consider the erroneous advice regarding 'all mouth and no trousers'. I'm afraid that I rather feel that it is 'all mouth and no trousers' because the idiom refers to someone boastful who cannot back up their boasts with actions in the, er, trouser department.
"By missing out the word 'no' the meaning is lost, as is the earthiness of the metaphor. With the recommended formulation at best you can say that the individual concerned is both boastful and wears flashy trousers, but flashy trousers are hardly a well-rooted idiom.
"Frankly, I had never come across the phrase without the 'no' in it until I encountered in the style guide, and it saddened me mightily."

Foolish, ignorant reader. Marsh considers the usual sources, and comes to the correct if rather wishy-washy conclusion:
"All mouth and trousers" probably came first, and in the interests of consistency, we shall keep it in the style guide. But let's not get too prescriptive about it: both phrases will have their adherents, and you should use whichever pleases you more.

As previously noted, the people who prefer the bastardised 'no' version often typify the original phrase.

(And thanks to the Guardian's own Nick Das for mentioning this humble blog in the comments to the above.)

The Economist's entertaining Johnson blog has also picked up on the debate. The writer there sadly seems to assume the bastardised version is the original. Of course, that organ has a shameful record on this issue.