Ian, take another re-read through my posting. All of the issues you brought up are addressed.

What I am suggesting is that you use a 230 to 415 step-up, single phase transformer to power a 415V, 7 hp, 3-phase VFD which powers the motor. Yes, this VFD is rated for 3-phase input, which is why I said 7 hp VFD instead of the 5 hp VFD you could normally use for this motor if you had 3-phase available. The VFD is larger (oversized) to accommodate the single phase input. No, the sales guy probably won't tell you that you can do this, but it already appears that he is not telling you the proper information to begin with.

Because you won't be using the full 7 hp from this VFD, it will operate from a standard 32 amp circuit. It should be drawing somewhere around 25 amps at maximum 5 hp load from the 230 volt supply. (The output from the transformer-side will be 14 amps).

Also, don't forget that if you use a phase converter, you would still need to use a transformer before or after a rotary converter to give you the 415V that your motor needs to see. This is why I am suggesting that a VFD is still the best alternative in your case.

A dual voltage motor would be nice, but all that means is that its internal windings can be reconfigured to run at 230 volts. This would eliminate the need for any transformer, but since your motor cannot be configured in this manner, you don't have a choice and must power it from a 415 V transformer.

If you haven't done so already, it might be helpful for you to read through a couple of my articles. They were not written for the UK audience, but the general concepts are the same. They won't tell you how to solve your problem, but they may give you some of the background information to help make your situation more understandable.
http://www.waterfront-woods.com/Arti...lectricity.htm
http://www.waterfront-woods.com/Arti...econverter.htm