Just received an email from Ryanair Car Rental that they're having a sale with cars from £1.99 per day. Book by midnight February 19th for travel up to 30th June.
I've used them several times and pretty much always found them to be the cheapest. Contrary to their reputation as an airline their car rental system is actually very good. They're middlemen rather than an actual agency and for the most part the cars I've rented have been from Hertz. Unfortunately I've already made my car rental bookings.