No regrets in the bedroom, says Iain Duncan Smith as he pops £1bn question to Rachel Reeves

"No."

The work and pensions secretary stands out from the crowd in the blazing sunshine of Netherton, wearing a long, wool overcoat best suited to the chilliest of days.

He led the Conservatives for less than two years during the then unshakeable popularity of Tony Blair.

Now he is trying to convince people to stick with the Tories in Dudley South, one of the key marginal Black Country seats that changed hands from Labour in 2010. As we pass care home Netherton Lodge a man in the car park yells out 'I would be ashamed to be you'. He then calls Mr Duncan Smith a word that is either slang for a person of wealth, or it's something that's not really printable in a family newspaper before adding 'UKIP's going to beat you'.

Undeterred, the group presses on to streets like Church Road and Monarch Way. It is something of a mixed bag. One lady on the doorstep delights him by saying she will definitely do as she always has and vote Conservative. A retired couple are not sure. But the man tells Conservative candidate Mike Wood it will not be Labour. Mr Wood says 'that's a start'.

One lady says she never votes at all. A young woman returning from the gym when Mr Duncan Smith approaches says she's leaning towards Labour.

But back to the bedroom tax. Mr Duncan Smith has presided over some of the most difficult decisions of the coalition government, in terms of cutting the welfare bill. His critics say it is ideologically driven. Some suggest he is hitting the disabled and the most vulnerable the hardest.

The Tories imposed a cut in housing benefit for council and social housing tenants who have one or more 'spare' bedrooms.

The Conservatives call it the removal of the 'spare room subsidy'. Labour says it's the bedroom tax. Mr Duncan Smith says it has saved £1 billion on the benefits bill. Others say it has driven people to despair and even suicide.

But asked if he had any regrets about the change Mr Duncan Smith says: "No. Not at all. What I faced when I walked through the door was a housing benefits bill that had doubled over the last 10 years of the Labour government.

"It was set to rise by £6bn. Let's get that in context. It was money that taxpayers have to pay all the time. Part of it was poor use of housing, some people living in homes they weren't occupying all of and others living in overcrowded accommodation.

"Labour introduced this policy themselves. They introduced it for people in the private rented sector who were social tenants. We've simply extended it.

"We're seeing the vast, vast majority of people settling up, sorting it out, more people going to work, working more hours. If they want to stay it's only fair on the taxpayer they pay."

Labour pledges to scrap the 'bedroom tax'. It will be Mr Duncan Smith's shadow Rachel Reeves first job if she takes over the department.

"I'd like to know how they're going to pay for it," he says. "It's going to cost them £1 billion. The only thing she has said is to spend more money. She's opposed every single change we've made."

The Conservative majority in Dudley South is 3,856, which is less than the Labour majority the Tories overturned in 2010. The incumbent MP Chris Kelly stood down after one term. Now Pedmore and Stourbridge East councillor Mike Wood will try to hold it.

"I'm very confident," says Mr Duncan Smith.