[MapProxy] Mapproxy WMS requests at some resolutions return an error

[Fiona Hemsley-Flint]

Hi,

I am fairly new to MapProxy, though I have used TileCache in the past 
and generally familiar with WMS etc, but I am having what seems to be a 
strange problem with resolutions.

In MapProxy I have a grid specified:

  bng-historic-overlap:
    srs: 'EPSG:27700'
    bbox: [0, 0, 700000, 1300000]
    bbox_srs: 'EPSG:27700'
    res: [1000, 900, 800, 700, 600, 500, 400, 300, 200]
    origin: 'sw'

Which is used by the cache:

   historic-overlap-backdrop-cache:
     grids: [bng-historic-overlap]
     sources: [historic-county-overlap]
     link_single_color_images: true

And the layer is defined as:
   - name: historic-overlap-backdrop
     title: Cache for historic-overlap-backdrop
     sources: [historic-overlap-backdrop-cache]


when I do a request through OpenLayers2 with a resolution of 1000, then 
an image is returned, but for 900, 800, 700 I get the following error:

     "Request too large or invalid BBOX"

And when I request 600, 500, 400, 300 and 200 images are also returned.

I can't understand what is going wrong, and I hope someone can shed some 
light on the problem?

Many thanks,

Fiona