Thoughts...

• No one knows the age of the linings. Sure they are BMW, but they could be 50 years old and 3 generations removed from the latest braking materials. I would tend to buy new brake shoes from a name-brand aftermarket seller that moves a lot of product, someone like EBC.

• The most likely issue is simply the cumulative absorbing of chemicals (including grease and oil) over the years. Maybe it's simply time for some new EBC shoes. 

• Still before tossing them away, let's get out on some high speed roads where there's no one behind you, where you can do emergency stops to bed the shoes in. You'll want to be going 50+ MPH and clamp on the front brakes in 1 sec ON/ 1 sec OFF pulses. Do that for several miles. Some of the really advanced linings from that period need to be HOT to really get grabby. 

• Often times, lacing a wheel after the fact will distort the drum. Most drums are trued after lacing. If you get this issue, you will most likely feel "pulsing" in the lever or braking effort.

Hope this helps.

Thanks @richard!  I think (but may have been my imagination) that they were getting a little better with a few miles on them BUT at only £49 I think I'll invest in some new shoes as per your suggestion and my 'hunch'

• Also try lubing the front brake cable with some ATF. You want to make sure the shoes aren't dragging the drum and getting over-heated between applications.

• Check the operation of the 2 actuation levers. On some vintage bikes, those steel shafts can seize into the aluminum brake plate.

• Sometimes people are afraid to blow out the drum and shoes because of the asbestos dust. But you can always wash those 2 places out with with plain, cold water from the garden hose in your front yard. (In your case, since the drum has already been cleaned just the brake plate.)