I've used the Nationwide Index before and found that it was grossly inaccurrate, for my area at least.(posted 8480 days ago)My partner bought his home for £49990 in July 1997 and it was valued again for a remortgage last August at £75000, the Nationwide quotes £57000!! By using county rather than town/area, this index fails to take into consideration the difference between affluent and poorer areas in the same county. In my town for instance, house prices have risen a lot, but 20 miles away in the town where I used to live, in the same county, you can buy a 3 bed semi for less than £10k (and they're not repo's!)
Even the house I had repossessed shows an increase in value according to the Nationwide index, despite it being in a very poor/run down area and was in fact in negative equity by many thousands.