Morning Gareth,
Had the same problem with Morini 125H wheels that are Grimeca cast and Hiedenau tyres (2.75 and 3.00) size. Ex race tyre fitter who does all my tyre fitting had 85 psi and lashings of lube and even then they were reluctant to seat. So, put tyres in a warm room to soften and cleaned the wheels and tubes of lube and started afresh after half a day. Still a fight and still required 85 psi to seat, but they did seat.
Conclusion: some tyres have stiffer tyre sidewalls than others, the tyre needs to be warm and as flexible as possible. Some wheel rims, especially the cast, narrower sizes have little space in the central well of the rim for both tyre walls and a partially inflated tube. Lots of use of a rubber mallet and bouncing the whole assembly on the floor, as the pressure is increased, appeared to be the solution. Old tyres that look OK, but are more than 5 years old need to be given the heave ho.
I know this isn't a panacea, but at least you know that you are not alone in experiencing this problem. OH! For God's sake don't use Fairy, I corroded a Borrani rim to powder, due to the high salts content!
Regards, Bill