Ok I jumped the gun a bit as it's early morning
I have implemented these hardcoded changes into my model and it works as expected though I am only interested in changing the behavior of related fields.
#on serializer
class MySerializer (serializers.ModelSerializer):
class Meta:
model = MyModel
exclude = ('my_second_model',)
my_second_model_id = serializers.PrimaryKeyRelatedField(source='my_second_model')
Steve