OP
The original armoured cable was put in by the electrician when we had the extension built. On the basis that the water feature pump and the lights are all powered by transformers, down to 12v I think, surely an extension to the armoured cable to provide power for a CD player and some fairy lights isn't going to cause an overload.You will be restricted to 13Amps as is if the Cable is connected to a Fused Spur, may be best to get an Electrician in to do the job properly Rather than leave it as a DIY bodge.