Why does the red line curve upwards? Doesn't the phrase "trend @ c 2.4% per annum" imply that it should be a straight line, and how has the 2.4% figure been arrived at?

It's a moving average and curving upwards still because of HPI of recent years.

errrmmmm..... no.

It curves upwards because the trend is exponential. i.e. each year the 2.4% increase is a 2.4% increase of a bigger number.

If you plotted the graph on a log scale the red line would be straight.

The trend has been arrived at by doing the calculation:

y = y0*(x^n)

where y is the price now, y0 is the price n years ago and x is the rate of growth.