I didn’t find anything in the manual when I was puzzling over this, but I looked only because things weren’t happening as I expected.  If things had worked properly the first time, I wouldn’t have thought that dealing in decimals would have necessarily needed to be in the manual.

On a slightly different note for those who deal through IG Index, it’s annoying that IG’s test system sometimes has minimum bet sizes that are different from live trades.

I’ve also found that IG’s treatment of an order generated by PRT which has a bet size below IG’s minimum (because my risk calculation required that in order to trade) for some securities places the trade at the minimum but for others rejects the trade on the grounds that my bet size is below the minimum.  This is presumably a bug, but no-one seems too bothered about fixing it.