Funding the purchase of a non standard build.

Littlelight

My husband & I have rented the home we live in for 5 yrs. We own a property in another part of the country and have paid off the mortgage. We want to buy the home we rent and the owner wants to sell it to us for £250,000. The problem is that it's a non standard build so we can't get a mortgage. We can raise £200,000 from the sale of the property we own, we don't have any debt and we've got a good credit history but we can't find a way to raise the extra £50,000. We have tried to get a mortgage from companies that specialise in non standard mortgages but the property type isn't listed on their lists. There are about 40 of the homes on our site, there are queues of people wanting to buy them and they sell for around £300,000. We have looked at unsecured loans and calculate that a loan of £50,000 over a 5 year period would cost us £140,000 which isn't acceptable. Does anybody know how we can borrow the £50,000 at a reasonable rate? We're both mid 50's.