An update that will hopefully help those still waiting:
Received email today that I needed to submit a new photo and also provided details of where my supporting documents should be returned. Emails were exchanged, new photo uploaded and application approved in the space of an hour. I was impressed by the speed and the service once the application gets out of the processing queue so this does seem to be the bottleneck.
Full timeline for me so far is therefore as follows:
Documents received (Royal Mail): 2 February
Documents received (Tracker): 10 February
Application being processed: 22 February
Photo rejected: 9 March
New Photo sent: 9 March
Application approved: 9 March
Common sense about responding to emails promptly and being polite in your communications obviously go a long way.
I do not know if my email to my MP yesterday made a difference; the processing time of two weeks is not out of line with what others have reported so perhaps it was just coincidence. However, for those still waiting with an urgent reason for travel it probably doesn’t hurt to request their help.