Graham and All,

here is the solution for this litte exercise:

easy:
  description: NOAA radiofax forcast chart showing wind and wave height in the western atlantic
  projection:
    proj: merc
    ellps: WGS84
    lon_0: -65.0
  shape:
    height: 1100
    width:  1728
  area_extent:
    lower_left_xy: [-4230000.0, -655000.0]
    upper_right_xy: [3790000.0, 4420000.0]

Lessons learnt: The Satpy parser is picky. Make 2 spaces indents and at least 1 space after the colon ":"  .
The safest way is to copy an existing entry in areas.yaml, rename it 'easy' and change the parameters.

The test overlay was choosen for its simplicity (pure black and white, no antialiasing) and taken from here:

https://www.nhc.noaa.gov/tafb_latest/atl24_latestBW.gif

Maybe you find more usesful stuff to overlay on this site. Attached a heavily reworked overlay from OPC.
It shows storm BARRA (HARRY) from yesterday. Some sites in the UK might have to repoint their dishes.

Cheers,
Ernst