I agree with Insuring it in the name of the "Executors of the late X"
It may also be worth condsidering doing a landlords policy as it could actually be cheaper than doing a normal homeowners policy. The reason for this is the amount of companies are limited by the fact your nephew is living in in and I assume he is fairly young so a lot of companies will want to charge a rate that reflects his age.
A Landlords policy is a set rate typically £147 for £100000 of cover. As previous posters have recommended try a local broker, preferably one who specialises in property Insurance. If they do offer a Landlord policy check whether they require a Tenancy Agreement to be in place as some require this (If one is needed you could always download one from the internet and fill it out with your nephew)