I'm doing some trials for approximating old styley invert aswell, but I can somewhat easily get my hands on Billingtons Demerara and light/dark muscovado.
Gonna try a 50/50 mix of Demerara/light muscovado for #2 and use it in a bitter that I am gonna brew tomorrow, using the more "authentic" inversion method.
@Northern_Brewer if you were to guess just between the thumb and the arse, how much baking soda per 100gram of sugar is advisable to use for neutralising? I usually make invert for one batch the day before brewing and would like to have some idea of what to calculate on.