Click http://news.bbc.co.uk/1/hi/scotland/2482543.stm
for a BBC news-page on the subject. There, it is suggested that the annual average in the 12 months to November 03 was 10.5%. The figure for Edinburgh, however, was 24.6%! Clearly, then, it depends on where exactly you are interested in. I'm no mathematician, so my approach is doubtless inaccurate, but I imagine the average increase over 18 months was probably about 15%.
Edinburgh is astronomical, the most basic flats in the city are way over-priced, but move out 5/6 miles and they are more reasonable. Like places in England some places are more desirable than others and the prices reflect this.