I’m building a Kent powered locost with a lucus alternator, I put a diode in line to stop the lamp glowing when running but it still glows. Could it be a duff diode, or the wrong rating. I cant remember what rating it is & its buried up behind the dash. Can I just whack another one in line in the engine bay alternator loom & if so what rating should it be

You shouldn't need any diode to stop it glowing. It should go out when the alternator output voltage rises to equal the battery voltage, which should be the same as the alternator output voltage when the engine is running.
First check should be a voltmeter across the battery terminals. It should read about 12-13V when the engine is not running and should rise a bit once it's running. If it doesn't the alternator is suspect,or your wiring is. HTH