Thanks to eddypeck.
The ISV - Idle Stabilizer Valve (or Idle Control Valve as it's sometimes called) is exaclty what you think it is, a vlave that stabilizes the idle of your car. So if on tick-over the idle is rough, whislt there are various reasons it's often advisable to look to this as the casue of the issue.
The ISV can be cleaned, this will often solve the problem.
Can be found on 16v GT and Non-Kjet 8v ... so PB engine code 1987 onwards.
(http://www.sjmautotechnik.com/trouble_shooting/ecupic/idlesta2.jpg)