Discounts not applied on PayPal Express Checkout (EC) Submit Order form ( echeckout/submit )

I've trawled the Issues but not found exactly the same problem.

I have tested a number of combinations and determined that discounts are not being taken into account during when using Express Checkout (EC).

PayPal WPS: Shipping OK, Discounts OK
PayPal WPP: Shipping OK, Discounts OK
PayPal EC: Shipping OK, Discounts not calculated --> therefore NOT OK

The problem is that the discount is not showing on the Submit Order page ( echeckout/submit )

Note: I have set the PayPal option to pass the order as a single line item. I've not yet tried applying the patch covered in #815482: PayPal Website Payments Standard: Convert negative line items to discounts

Also note that I am using the uc_discounts_alt module to apply codeless discounts.
I don't know whether or not this has a bearing on the case.

According to the PayPal agreement for PayPal Website Payments Pro, in addition to having the WPS payment option as the Credit Card payment method on the checkout page ( cart/checkout), I must support the Express Checkout button on the View Cart form ( cart ).

Please can someone advise me what I need to do in order to have the discounts applied in this scenario.
I am willing to make a donation to get this problem fixed, as it will a) save me time. b) benefit others who also have this need.