How can I increase the amount of gravy I can make from one roast?
If you want more gravy you need to add more liquids and natural flavours.

Better if you have bone but . . . no bone:
Before you roast your beef sear it, so it has good colour when it comes out, deglaze the pan/baking tray with water/red wine/white wine or Madeira one of me faves and put that to one side. (in the gravy stock pot)
Add onions, carrots and parsnips in with the beef, and touch of water, olive oil and salt. Don't go mad with the water you're not poaching the beef!! Just enough to stop the veg burning - 5mm in the bottom (top up if needed).
 When the beef is ready (55C internal temp = medium rare more here) take it out of the tray, put the root veg to one side and and add the remains to your gravy stock pot. 
Sear the bottom of the roast with a little oil (it'll be a bit soggy from sitting in the veg) then put it aside to rest (5-10 mins if you're in a hurry 15-30 for 3kg will come out lush!) deglaze again adding the resulting jus to your stock pot, which now had a wealth of flavour.

Use that stock pot to create a beast of a gravy! Make it in the usual manner, if you keep the onions in they'll make it thicker and tastier - a stick blender with give you a smoother texture if that's what you like!