Found this, published in 2005:
Private school fees for day pupils have increased at more than 3 times the rate of inflation over the last 20 years, according to new research from Halifax Financial Services.
Key Findings:
The rise in private school fees for day pupils has increased by 3 times the rate of inflation (103%) over the last 20 years.
Average annual earnings in real terms (ie: after allowing for retail price inflation) have increased by 48% over the last 20 years. In real terms private school fees have risen by 129% in the same period.
Depending on where in the country a pupil is educated, the cost varies significantly by region.
Also this on another site:
Costs are substantial, have tended to rise faster than other forms of inflation in the past, and are likely to continue doing so in future....assume inflation of 6 per cent per annum
And then this:
Using average day and boarding school fees and, assuming fees continue to rise at 5 per cent annually (rises averaged 7 per cent annually over the past three years),
So, 7% seems to be about right at current rates.