The Finance Minister has been a vital and much-needed driving force in the backrooms of the Beehive where ministers and departmental officials thrash out the policy detail so that National's reforms actually work in the way intended.
In particular, Mr English pushed hard for meaningful welfare reform.
His other major target is state housing.
The latter is a fundamental component of what remains of last century's welfare state.
It is a safety net.
It is a Labour Party icon that has somehow survived National's attempts to dismantle it.
Mr English has long wanted to replace it with a more market-oriented system which, in theory, should be more responsive, more flexible and thus more cost-efficient when it comes to meeting the needs of both tenants and landlords than the current provider, Housing New Zealand, which enjoys a state-sanctioned near monopoly.
But constructing such a market seems to be proving to be far more difficult than envisaged.
It has taken four years to reach the point where the Prime Minister could release broad details of the plan.
What has emerged is not that politically palatable - especially the transfer of housing stock to private providers.
Mr Key has plenty to worry about. Wading through the ever-growing pile of Cabinet papers advising ministers of progress, the reader is drawn into a world where theory reigns supreme and it is assumed everything works in the way the economics textbooks say.
There seems to be little cognisance of the danger of policies having unintended consequences - ones that might well hurt National.
What is looking likely is that officials are developing a market for social housing which will require heavy subsidies to attract entrants, thereby defeating its purpose.
Mr Key and Mr English will be left trying to explain why the current system which seemed to work adequately enough was dumped solely because those promoting its replacement were devout believers in the notion that the only solution to a problem is a free market-based one.
Such a major policy failure will also open Mr Key and Mr English to charges that the whole exercise was cover for privatising a major component of the country's social services.
What might be described as Bill's Big Experiment has several forces driving it.
These include serious questions of efficiency springing from Housing New Zealand's effective monopoly, forecasts that the increasing amounts being spent on income-related rents and the accommodation supplement are fiscally unsustainable.
A market model of delivery was seen as bringing more contestability by opening up the provision of social housing to more players.
But to work, the model entails selling thousands of houses managed by Housing New Zealand to both non-profit and (probably) commercially driven organisations which will take on the role of ''community housing providers''.
Those seeking housing assistance will increasingly be referred to a community housing provider and might well end up with a home that once was a Housing New Zealand property.
Mr Key's concern is clearly that the reform will be viewed by some as National using the model as an excuse for an asset sale.
That prompted him to use this week's speech to place a few stakes in the ground limiting how many state houses will be sold to community providers and how soon.
The constraints apply only in the short-term.
Sure, the numbers up for sale are markedly lower than the 20% to 40% bandied around in Cabinet papers written by senior officials from the Treasury and the Ministry of Business, Innovation and Enterprise.
Mr Key's assurance, however, that Housing New Zealand will still have 60,000 houses under its administration in 2017 actually amounts to a 12% reduction in stock.
Assuming an average price of $260,000 applies to all of the houses in that 12% cut in stock and they all sell at that price, the Government would reap about $2 billion.
The Government will not receive anywhere near $2 billion. Community providers simply do not have the cash.
Providers have consequently been lobbying hard for a combination of tax concessions plus government help securing loans so they can purchase houses which will be thrust at them by the Government at what will be knock-down prices.
The providers essentially have the Government over a barrel. If the model is to work they need to be in the market with houses to tenant.
If only a few new providers enter the market, the policy will be deemed an embarrassing failure.
If those houses are priced too cheaply, National will face a storm of criticism for being loose with taxpayers' money.
The Government is hoping providers will enter partnerships with commercial backers with money to invest in the bottom end of the rental market.
But why would investors risk placing their capital in the most problematic end of the market for rent arrears and property damage?
All this is a headache Mr Key does not need. But what will have worried him most of all is the unusually frank language of officials in one Cabinet paper.
It says the change in the delivery mechanism will have to be gradual and staged over 10 to 15 years.
Moreover, the officials warn that it is ''highly unlikely we will get the reforms completely right the first time ... Accordingly, there will have to be some experimentation, some of which will work and some of which will not''.
That is OK for bureaucrats. They get a second chance. Politicians don't.
• John Armstrong is The New Zealand Herald political correspondent.
