To follow up to my previous comment, I spoke to IG on the phone and they confirmed that demo accounts do not handle corporate actions well, including stock splits.
I feel this should be made abundantly clear without having to dig through the forums or call customer service. I was trialling some trades I'm thinking of opening on my live account, and seeing a sharp loss on them on my demo account with no explanation given at the time does not fill me with confidence in leaving long dated positions open.