Are you reffering to post #83 and following explanation by IG? If not give me post no.. Apparently this guy was trading the FTSE June 10 cash contract and not the rolling daily. Incredibly easy to investigate the real value and spread of that contract during the time in question. If it turns out that IG is right in what they are claiming in post #83, I can't see that they have done anything wrong in reverting the trade.
"In relation to how IG make their prices on their FTSE contracts, I can confirm that IG take the mid-point of the underlying future market (near quarter), make an adjustment (from this level) for ‘fair value’ and apply their spread to give their Cash, May, June and September prices. The ‘fair value’ adjustment accounts for interest and dividends. Ordinarily, this spread would be 1 point so, for example, if the underlying market is quoting 5179/5180 the mid price would be 5179.5 with IG’s quote reflecting 5179/5180. However, as previously mentioned, in reaction to events in the financial markets on the evening of the 6th May, the spread in the underlying future market was widened to 800 points giving, at that time, a market bid/offer of 4000/4800 with a mid-price of 4400."