M500/M1000 (with connections under the endcaps): 9 volts, DC, center positive, 1.3A. Outside diameter = 5.5mm; inside diameter = 2.1mm.
The M1001 (includes units labelled as M1000 but with connections at the back) and the M400 PX (aka Pinnacle HomeMusic): 5.0 volts DC @ 1500mA (or 1.5Amp) connector uses 2.1 inside diameter and 5.5mm outside diameter. Center positive. Take care to ensure that the replacement PSU has a straight edged connector. The elbow connectors that are now common will not fit inside the recess. Ask the online webshop about this specifically, or visit the real shop. This is true of all units with the power connector in a recess. Apparently, Maplins sells the straight pin connectors separately.
For US Purchases: You can purchase replacements for the m1001 at: http://shop.roku.com/accessories-all (The Roku Streaming Players power adapter, does work with the Soundbridge m1001)