I have fitted a small denso alternator with 60mm pulley wheel, when engine revs drop bellow 1000 rpm the alternator stops charging. Then when reved up it starts charging again. Would a 50mm pulley sort it out.

Some alternators will only charge above certain revs

Should equate to increasing the revs by around 20% anyway I didn't think you did low revs much!

Might get stuck in traffic. will look a wright walley sitting there revving my car. Maybe thats why all the boy racers do it.

What is actually happening to the battery voltage?
You need to check the voltage under various loads and rpm.
No load engine switched off should be 12.5 to 12.8v
No loads engine idle voltage should be at least equal to voltage with engine switched off.
No loads 2000 to. 2500 rpm, expect 13.4 to 14.9v
Head lights on. 2000 to 2500 rpm.expect 13.2 to 14.8v
Headlights on idle speed 12.4 to 14.8v

Is the warning lamp of the correct wattage?
Is the alternator new or second hand?