The "1W" amplifier from Maplin uses the LM386 little power amplifier IC. It cannot produce an output of 1 Watt unless the output is an extremely distorted square-wave.
Its output at clipping into an 8 ohm speaker with a 4V supply is only 0.06W, with a 6V supply its output is only 0.2W, with a 9V supply its output is only 0.45W and with a 12V supply it gets very hot and has an output of only 0.53W.
Driving a load of 4 ohms or 16 ohms results in less output power.