Yahoo Finance - Data inconsistencies between historic and current data
You might have better luck using Quandl as a source.  They have free databases, you just need to register to access them.  They also have good api's, easier to use than the yahoo api's Their WIKI database of stock prices is curated and things like this are fixed (www.quandl.com/WIKI ), but I'm not sure that covers the London stock exchange.  They do, however, have other databases that cover the London stock exchange.