The Sketch: Dave's at B&Q so Maria doesn't do it herself

Shirtsleeved, in the packed atrium of B&Q's headquarters Dave – and this was very much a "Dave" occasion – was at his frankest. "Look," he said, asked if the debate on Scotland's independence wasn't a bit messy, "that's a very good question. I'll lay my cards on the table." We tensed. "I passionately believe in the United Kingdom."

Appearing amazingly candid while saying something entirely predictable was very much part of the Dave shtick on his trip to Eastleigh to back the Tory candidate in the forthcoming by-election, Maria Hutchings.

Asked whether the by-election mattered since it was between two parties in government, he again decided to "lay my cards on the table". The "most important thing" was "who knows the area, who is going to stand up for it … and that's what you get with Maria".

It would have been more exciting if he had said: "As far as I am concerned, Scotland can sod off. And I need a win in Eastleigh to bolster my leadership but quite honestly, given her hostile views on Europe, gay marriage, abortion and whole lot of other hot-button issues, she's going to be exactly the sort of rebellious backbencher I could do without."

But that wouldn't have served the purpose, which was to emphasise Ms Hutchings' local credentials (though she originally hails from outside Eastleigh and has been accused of boning up on it via Wikipedia) and that she has four children (the poor Liberal Democrat candidate, Mike Thornton, has only one). While ensuring that she didn't answer questions on policy.

And it worked. By the end, we could have recited her attributes as a "local mum" in our sleep. He was helped by the setting, with its happily captive audience surrounding him on three levels and decorated with B&Q slogans – like the convenient "Handy prices to help you do it". Eat your hearts out, Office for Budget Responsibility, with all your gloomy talk of rising costs!

The toughest question was probably whether he was a member of the B&Q customers' loyalty club. No. He was "famously" bad at DIY "but I'm very fortunate to be married to someone who's not just artistically brilliant … but who can knock up a set of shelves in no time". Then he mysteriously added that he needed the compliment on the record because "I didn't do so well on Valentine's Day this morning". The B&Q staff were too polite to ask what went wrong.

Once, by-election candidates held daily press conferences, with visits by party leaders a valued bonus. On Wednesday, Cameron was like a human shield, ensuring the limelight on his would-be MP was not too harsh. She did speak at B&Q, describing how she had fought a greenfield development here, opposed a gravel tip there.

But then, introducing her, the Prime Minister had declared: "She's a local mum, four kids and she's had a great business career. She lives here, works here and she will really put her back into working for you."

Local. Mum. OK, Dave, we got the message.