Thanks for the replies guys, I think it's only one of the valve's gone, basically, when I turn it on, the red power light comes on, but when I try flipping the standby switch the green light doesn't come on, and there's no sound at all apart from background fuzz! ;(

In all honesty I've found it a little impractical for practicising too, it doesn't sound too good when it's quiet, it takes a while to warmup/warmdown and as has just been proved, it's potentially unreliable! ;(