I don't have an IG index account so I may not be that much use to you here.
When you say "but I know for a fact" how do you know this? Do you have some other live price feed? Also I know most of the spread betting firms say, prices are 'indicative' which basically covers them for things like this.
It would seem that if IG are quoting prices that are below market value and they could potentially "catch up with themselves" then that could be an opportuinty to spread bet them? This is not advice or a suggestion, merely an observation.
Someone else with an IG index account might be able to answer your question better than me.
On a side note, you commented
about CMC markets. If your after AIM shares then the new CMC platform is not for you. Their old market maker platform had way more instruments than the 1250 or so they now quote. To be honest I can see them losing alot of clients when the market maker platform is discontinued. I personally try and stick to FTSE350 stocks, mainly because they are more liquid and can be less volatile than AIM. But if you prefer AIM and it suits you then stick with it. I think you are probably with the best provider for AIM stocks.
May the markets be with you!
The Spread betting beginner