What rent should I charge married couple lodgers in one room?

I have a very nice large double room (with double bed) with TV and wireless broadband that I let out to lodgers now and then. Up till now it's always been a single person having occupancy of the room, and I've been charging £80 a week. My current lodger moves out in June and I've had an enquiry from a married couple interested in the room. What should the rent for a couple sharing a room be? On the one hand, assuming they're both out at work at the same time, they would be using the same amount of heating and lighting in the room as one person. On the other hand, they will be using twice the amount of hot water for showers, etc. What do people think would be a fair price? Assuming this would take me over the £4,250 limit in this tax year, what percentage of tax would be taken off the excess income?

Many thanks for your help.