Thanks for the suggestions.
I have at present nearly 8 inches (after re-measuring!) of absorbtion behind, and there is definitely a better response with the speakers, bass wise, now that I have them backed up close but there seems to be a little lift in the upper bass/low mid range now.
Do you think it would be prudent to make a hanging 'cloud' bass trap above the alcove, and is it possible that I am getting ceiling to floor anomolies now?
The speakers are definitely beyond the half way point on the vertical axis. At present I do have two ceiling corner traps there (see 1st photo) but I do cherish the little window for natural daylight!
Barkley