AX 2012 Sales lead time. Trade Agreement vs. Default order settings

Hi, We'd like to use Delivery date control = Sales lead time.

I assumed that if a sales price trade agreement exists with a lead time of zero, that the system would use the Sales lead time on the Sales order tab of the Default order settings - but this isn't happening. If a sales price trade agreement exists with a lead time of zero, the sales lead time is treated as zero.

Any work-around? Is there a setup parameter I've missed?

Thanks

TIm