£5 first class stamp?

Two weeks ago the Royal Mail put up its stamp prices - they do this annually, and I wondered just how much they had gone up in price over a longer time period.  So I put together this chart showing the price of a first class letter from 1840 to April 2011.  A stamp cost 1d in 1840 and did not go up in price until 1920, that's several generations.  If you found an 80 year old stamp behind the settee in 1920 you could still put it on a letter and get it posted.  Try doing that now.

The chart clearly shows something very wierd happening from around 1970, the price goes exponential, and since 5 years ago it is going vertical.  The values for the last few years are an average of the regular and large letter prices.  One of methods used to disguise price rises is to create a larger number of products or services and vary the price structure, this makes comparison impossible, so I have just assumed that equal numbers of regular and large letters are sent.

The question is then, as prices are now increasing exponentially, how long before our first £5 first class stamp?