Huhne's '£40k blog cost' explained

Where did Chris Huhne get the figure of £40,000 for the annual cost of David Miliband’s blog? I thought I’d email to ask him. And he replied. So full marks there. 🙂
It all comes from a parliamentary question, answered by DEFRA’s Barry Gardiner on 5 June:

The Secretary of State writes his own blog. Two staff in Defra’s Communications Directorate – at Grade 7 and Higher Executive Officer grades – have integrated the blog into Defra’s website, and continue to oversee operation. For the two weeks following the recent ministerial changes, approximately 30 to 40 per cent. of their time was spent on work in some way connected to the blog. This is expected to decrease. The blog promotes a new and more direct form of communication between the public and the Secretary of State.

Apparently his researcher ‘merely took the salaries of the grades specified to get the result.’ I can’t lay my hands on the DEFRA pay scales, but looking at other departments’ data (eg DfES), a Grade 7 salary is probably £40k to £45k per year, and an HEO salary around £25k.
So how do you turn those salary numbers into an annual total of £40,000? The best I can suggest is that they’re taking the total salary for those two people (let’s say £67,000), multiplying by the top end of the time estimate (ie 40%), and adding 50% to cover ‘overheads’. (That comes to just over £40,000. Try it.)
But let’s look at those figures another way. Two people spending ’30 to 40 per cent’ of a ten day period is a total of eight person-days. Even adding 50% for overheads, I make that a setup cost of just over £2,000 – after which it probably becomes normal website maintenance, and almost impossible to quantify separately.
As ever, your calculation is entirely dependent on your guesswork, your assumptions, your extrapolations… and given that this is Westminster, your party position. Always remember this is politics, not mathematics.
Many, many thanks to Chris for replying.
Update: I’ve had a subsequent email from the LibDems’ Daniel Wilson, confirming: ‘PCS civil service wage statistics show that the two civil servants who work on Miliband’s blog cost up to £61,457 p.a. (for the senior member) and £40,500 p.a. (for the more junior member). As they spend up to 40% of their time on the blog, this equates to £40,782.80 p.a. in staffing costs.’ So even if those PCS figures are valid – and they aren’t too far away from my ‘add 50% for overheads’ numbers, the calculation is based on maximum figures across the board. Politics not mathematics, as I said…
Update, 25 July: grateful to Guido Fawkes for referring the traffic to my humble blog… but I didn’t ever claim it ‘costs the taxpayer a pound a word’. If anything, I think I did a reasonable job of undermining Chris Huhne’s initial claim. And Miliband told the (Newcastle) Journal on 17 June: ‘We are trying to bed down the ongoing administration costs as we speak but we know that they are going to be less than 10% of an existing junior civil servant’s time.’ Even applying the ‘add 50% for overheads’ rule, you’re talking £3,750 pa.