Lyns - the motilium can sometimes increase blood pressure (it's used to treat the low blood pressure that is a side effect of Parkinson's medicines), but it works in a very specific way, so it depends on the mechanism of your other medicines and your high blood pressure. Maxalon is another medication that can increase milk production - though it is older and used less than motilium.
If your Dr says "no" then have a talk to Rodney Whyte at Monash Medical Centre, who is an expert on medicines and obstetrics/breastfeeding. His phone number is (03) 9594 2361.

Can you hand express out the last bit after expressing - even if you don't keep it, everything you get out is sending a signal to "make more".