You also need to take into account if you rent the BTL insurance. I pay £30 with Nationwide, but as soon as that is finished I am changing over to Drayton Insurance as they were recommended to me by my letting agent and charge about £13 a month for my other propety. You also need to take into account the charge that a letting agent will charge. We pay 10% plus VAT but one of the big companies had a bronze, silver and gold service, different rates, I think starting at 12%, the gold being for those that live abroad and was expensive. They also sounded like they would send someone out at the drop of a hat, which of course you get charge for, plus any work. Our agent was a builder and is happy to look at stuff without charge and work out what is the cheapest option.
good luck