Hello Ben,
Good question in my book, and one which suggests - and I'm pretty certain confirms- that the car is fitted with non standard wheels. You really need to have this confirmed or refuted and if they are non standard then you would need to inform your insurers of this fact, otherwise you run the risk of possibly being declared uninsured - not a place you want to be.
If it were mine, meanwhile I would go with a reasonable compromise pressure as if they were 17s. Trevor's suggestions seem helpful, but I am surprised about the higher than expected unloaded figure of 36psi for the front if I'm reading it correctly.