I strongly recommend Cruisekings,
http://www.cruisekings.co.uk/ . They are pretty well unbeatable on price and are also incredibly nice, helpful and super-efficient. We have booked 2 cruises with them and they have been superb.
I haven't sailed with Cunard, but we have just booked a P&O cruise for October 2008 and all the suites and balcony cabins, the cheapest inside cabins and the highest grade of ordinary outside cabin are already full, though it only went on sale a week ago. Cruisekings managed to get us the very last cabin in the grade we wanted!