If you are choosing between NAD and Marantz at the £300 price point (C326BEE/PM6004), they are pretty evenly matched. The NAD has a big, slightly warm sound with lots of punch. The Marantz is a slightly cooler headed performer, focused more on detail and quite a smooth listen. Both are excellent amps. Personally I love the NAD but you need to listen to both. It also depends very much on planned partnering equipment.
As PP says there are plenty of other options too. I'd certainly consider the Denon PMA720AE, the new Rotel RA10, the Cambridge Audio 351A or (if you can stretch) the 651A. Then there are the new amps from Pioneer and Onkyo, which I haven't heard yet but which should be very worthy contenders knowing what the two brands are capable of. Also from Onkyo, if you fancy more flexibility, is the TX8050 receiver which is really rather good.