Sorted it!
A combination of the diode (thanks BMNET) and experimenting with various capacitors across the motor terminals fixed the problem - I suspect a particualrly noisy motor.
Thanks to all who answered.
The output is driving a 12V SPST relay used to turn a 12V motor on/off. I have already added a flywheel diode across the relay coil for back EMF protection.
Iv'e spent ages trying to solve what should be a ridiculously easy problem but it's got me stumped.
A standard 555 monostable circuit, output drives a small relay that controls a small motor - it should turn on for 2.5 seconds when the 555 is triggered but just turns back off again - 555 doesn't...
