The Pension Service figure appears to be correct. For starters, the maximum Savings Credit for a single claimant was £14.79 – so they couldn’t receive any more than this.
I tend to calculate Savings Credit awards like this:
Maximum Savings Credit: £14.79
Qualifying income: £117.65
Threshold: £77.45
Excess income: £40.20
60% of excess income: £24.12
Amount A is lower of £14.79 & £24.12 = £14.79
Appropriate Minimum Guarantee: £102.10
Excess income: £15.55
40% of excess income: £6.22 = Amount B
Savings Credit = Amount A minus Amount B
= £14.79 minus £6.22
= £8.57
QED
Of course, this was the figure before the April up-rating – the current award of Guarantee Credit will be marginally different.