Alan
Before you have another go, does your 48448 have the updated .eng file? This is at file ID 1262 and makes it a slightly better steamer.
My technique was to put the regulator at 100% from the off after picking up the banker and leave it there (like Terry, I left her brewing until I got over 200 psi). After setting off at 75%, by the time I got to the 'welly' message I was down at 20%, then I applied the sander and in simple terms I increased cut-off by 2-3% for every mile an hour I lost in speed until I was at about 70-72% cut-off. My minimum speed on the bank was 6 mph and after you cross over the CLC it gets much easier.
If you slip, you've had it - you might as well forget it, so make sure changes in cut-off and regulator are reasonably smooth and keep the sanders on. I haven't played with the physics on my wagons, and according to Conbuilder the consist without banker weighs 682.8 tons.
It took me ages tinkering with adding or removing wagons to get the load at the point where the activity is challenging but possible - I didn't want it to be too easy!
Easy-Peasy
Terry
All the world loves a smartypants!

Seriously, following Matt's timely seasonal reminder I'd like to thank Terry here on the forums for such a fantastic effort creating NWE, and to promise everyone else that I have plenty more NWE 'orrible activity ideas in the pipeline - shock horror, I'm even working on a modern-day one...