Alright guys
I recently sent back some £30 speakers to Scan to test, explaining that the problem was a sporadic one that only occurred after several hours of continuous listening. Whilst playing the sound to the speakers would just suddenly start to die. The volume gradually diminished without touching the volume control until it faded completely. The source (a laptop) was checked and was working fine. All the connections were checked and I confirmed that all the wires were plugged in. Power was fine and the blue light was on. But no sound.
After leaving the speakers for a while the fault seemed to disappear and they worked again. However, when the fault reappeared I RMAd the speakers for testing, explaining the strange nature of the problem.
Now Scan have decided to charge me £30 + VAT (i.e. more than the cost of the speakers) for their diagnosis of: not faulty.
When I enquired as to the testing process it became apparent that they hadn't been tested properly. I re-explained the problem and said that they would probably need several hours of continuous play time in order to find the fault. Despite this (and having had the speakers in their possession for over a month now) they only tested for one hour. Clearly not enough time to assess the fault as described at the beginning.