ABS faults are common on 307s ten years and older.
The problem is usually down to dirt getting behind the mount point of the sensors - usually the rears - and they push it too far from the hub.
Its easily sorted by taking off the wheels, unscrewing off the sensor and cleaning the mount point with a wire brush. Takes about half hour tops.
If that doesn't solve it then a Peugeot Planet diagnostic is the way to go. It checks the voltage and activity at the sensors.
Peugeot charge £80+VAT for a diagnostic but many Pug owners have the same equipment and do it for beer money. Have a look on this map and find your closest member:
maps.google.com/maps/ms?msid=214397633218647067111...e
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