The relay isn't a starter relay, it is the start inhibit relay. All the starter solenoid current passes through the Ign switch, as opposed to the relay making a direct battery feed.
iirc, it is the outermost relay behind the left hand headlamp.
The starter current passes through the ignition switch, then the start inhibit relay main contacts.
The relay is energised by the ignition switch sending power to the gearbox selector switch, which transfers power to the start inhibit relay coil when the selection is either Park, or Neutral, thus closing the main contacts.
The dashboard indicator should also show the selection the gearbox selector switch is making.
All the Series 1 wiring diagrams are in the, below, linked thread.
viewtopic.php?f=8&t=10268
As citroenxm says, the fault could be either the gearbox selector switch, or the start inhibit relay.
Shorting out the main contacts of the starter inhibit relay would allow the Ign switch to send power to the starter irrespective of the gearbox selection. Make sure the gearbox is in Park before trying this.
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