Wall Street spread bet DFB exact underlying market?

[John G]

What is the exact underlying market that the Wall Street spread betting DFB market on IG is replicating?

I'm trying to backtest some ideas on this but any price data sources I see don't match the IG prices, not even really that close. 
Is the the E-Mini Dow Future (YM) ? Or some sort of derived  price from the actual DJI index? 

If anyone knows and can also point me to where I can purchase historical 1 min data it would be a big help! 

[Outlier]

Good question John. I am in the same position back testing various systems on Multicharts with IQFeed as a data source.

Did you ever get an answer (obviously not here) or find a solution to your back testing.

I am using E-Mini Dow Jones $5 (on IQFeed it is @YM#C) as a proxy but it is way out from IG's Wall Street.

IG appear to follow the underlying index (DJIA) whilst it is open, but not sure what they do outside the index session times.

Any more info would be appreciated.