When you place a spreadbet, you're not actually placing a bet on what the FTSE100 index does, it's a bet on what IG's FTSE100 instrument does. This means they can offer their market whatever hours they like. As the FTSE is driven largely by what happens on the US markets, it makes sense that they keep their book open while other markets are trading. This means you can continue trading but it does prevent anyone betting on the opening gap the next morning at 8am by simply waching what the US and Asia do and taking that direction before 8 here.
Ah, ok. Thanks for the clear explanation. I suppose this is also why the buy / sell prices don't correspond to the actual share price reported on other sites? I mean, it is approximately the same, but sometimes the actual share price is above the buy (i.e. not even within the spread being offered by IG Index)?
Yes. Prices on the Rolling SB instrument for the index will be closest to the underlying index's value but the spread will widen and narrow through the day, and the bias, whether the SB instrument price is a little above or a little below the underlying will also vary. The SB futures instruments, e.g. the FTSE100 December SB book, will be much further away from the current underlying - currently, December is about 60pts lower.