UK MH insurance for overseas residents

You are going to struggle because a) you don’t own the MH you wish to insure and only the owner/keeper can usually insure it, plus b) the MH is already insured and you cannot normally have two policies for the same vehicle.

I’m sorry I can’t offer anything constructive but there are a couple of similar threads here concerning insurance for NZ people.

I can’t help either - other than suggest your son tries as widely as possibly to  find an alternative insurer that might possibly allow him to add you as named driver - and that will mean him making a lot of phone calls. 

Not much help but could you transfer ownership/keeper to yourselves c/o your sons address and insure yourself?

We bought a car when visiting our daughter in NZ for 6 months and got insurance via NZ AA. Of course vehicles don't/didn’t have to insurance so might be different. Again she puts us on her car insurance if we're not hiring for ourselves. We often hire campervan/car to explore independently. 

Its not the proper solution but a work around which seems bizarre and may not be legal. Out of interest can he add you to his car insurance? Meaning its tge vehicle type rather than address?

I agree with eurortaveller hours on the phone. I hope you can resolve.