CMSMS Simplex Theme - How to auto-adjust banner height

[tidalstudios]

Hi
I have completed a clean install of CMSMS and I have applied the Simplex Theme to all pages. All is well and the responsiveness is working. HOWEVER I can't figure out a way to make the height of the grey background on the banner image 'auto shrink' to fit the height of the image as the width of the screen decreases. Essentially, the banner image reduces but the overall height of the div seems to remain static and leaves an unsightly grey box below the image.
It can be seen here (at my 'sandbox' site): http://www.theoldplank.co.uk/
I have not altered the coding at all; this is straight 'out of the box'. I have applied the theme to an active site and I am having the same result (it can perhaps be seen more clearly here):
http://www.1stcompletehouseclearance.co.uk/
Am I asking for too much or is there a way to make the overall height of the banner reduce with the height of the image?
Regards
Paul

[Dr.CSS]

Most times it's a matter of giving them a height then a max-height:...