Well it's nice to know I'm not alone. My supply seems to be increasing nicely.
My totals for the past 3 days:
Monday - 545ml
Tuesday - 590ml
Today - 590ml so far - still have another before bed express to go. Not bad for day 7 I reckon!

I haven't hired a medela pump. I have a different electric pump at home. When I visit DD2 I use their pump there to express a feed. My pump isn't as fast, but it is working at the moment. Hopefully it's only for a few more weeks...