On the diesels, the MIL will not go out even when the fault is fixed. The throttle position sensor went on my 45TDi, causing limp home mode. (Revs fixed at 1300)
I replaced the sensor, car drove normally, but the MIL remained on for 6 weeks until I had it reset by a independent with an ACR4.
The pre '99 cars would normally only light the MIl whilst the fault was present. Switching the ignition off and on again resets it. These cars are not EOBD compliant.
I believe the later cars are like the L series diesel so the light will never go off of its own accord.
Indeed, the EOBD standard requires it to be manually reset.