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It has been a difficult summer for Iain Duncan Smith. I understand that his long-standing arguments with the Treasury over funding his welfare reforms were essentially resolved at the "political" Cabinet at Chequers three and a half weeks ago.

The Chequers deal seems to be pretty much last Friday, and the FT yesterday.

IDS will have to find savings of £10bn by 2014, and the Treasury will allow him to take from these a ring-fenced pot of £2.5 or £3bn to pay for the up-front costs of his radical reforms.

IDS must be an extremely difficult colleague for Cameron to handle. Not only does he carry extra authority as a former leader, but welfare reform has become a personal crusade. He is unlikely to be restrained by other ambitions. Welfare is THE big thing for IDS, the one reason for staying in politics.

Many observers believe that of all senior ministers IDS must be the leading candidate to resign. That would be a big blow to Cameron since IDS would be a dangerous backbencher, a respected and admired figure who might rally critics on the Tory right.

So it was important to keep IDS happy. That is why Cameron intervened personally and persuaded George Osborne to keep IDS onside.

But IDS is still not entirely happy. DWP sources say he has been upset at the way Cameron sometimes makes announcements on welfare without telling him.

I'm told, for example, that when Cameron went to Manchester last week to announce a crackdown on benefit fraud, IDS was annoyed not to be told about the trip, though he had no quarrel with the policy.

Today's stories about restrictions on universal benefits, or so-called middle-class benefits, seem to come more from Liberal Democrat elements of the coalition than the Conservatives.

Indeed the junior Lib Dem pensions minister (and welfare expert) Steve Webb had a Parliamentary question last April in which he asked how much would be saved by raising the qualification age for from 60 to 65.

It showed the way the Lib Dems were thinking.

And the answer to Webb's question? £600m in the first year, though lower sums in later years and the female retirement age gradually goes up.

Indeed the Lib Dems pledged in their manifesto to raise the age for the allowance to 65 so as to pay for the payments to be extended to disabled people.

The latter idea is no longer a possibility of course.

Today marks the 100th day of the new government. What has surprised me is its sheer energy - particularly after ministers were involved in years of exhausting election campaigning - and the radical nature of what its trying to do.

But as we have seen with Michael Gove's free schools policy and with IDS's welfare plans, and many smaller measures, radical policies cost money.

And money is not what this government has got.