Where can I get accurate OHLC data?

I need OHLC data for the FTSE 100 going back as far as possible. It needs to be downloadable to Excel. Yahoo offers this but the O is the same as the previous day's C, which is no use to me. Does anyone know where I can get this data with the correct O?

Define correct open.....

The open on the index value is an issue if all the shares haven't traded. I understand the openning auction was designed to overcome this however you may want data before this came about. Getting futures data will give you a tradeable price at the open, but it won't match the open cash price.

Basically I'm looking for the first price that's posted for the FTSE when it opens at 8am.