Julian - if I had responsibility for you then I would send you straight off to my physiotherapist - who has specialised in back pain.
She'd look and feel and prod and ask: "if this hurts" and most likely send you off for an Xray of your lumbar spine. Then it would be said: " come back and see me again - so that you can take off your clothes, lay down on my work bench, and I can massage your back".
But the important thing she will do is to tell you how your back works and what you must not do to it to abuse it.
As for the work - and the horse riding - well of course in both there are potential causes but how can we over the Internet do anything but make comment - some of which will undoubtedly be irrelevant.
As for your continuing to work, well if you don't get the cause of your present aches sorted out then the liklihood is that the pain will only get worse.
But one thing I do know - is that if you are putting your lower back to significant strain then one thing for you consider is an elasticated sports belt which fits around the waist. It will give support to your muscles.
Remember: posture is king.
PS Just to create a laugh - when really there is nothing to laugh about - I get back ache first thing in the morning after waking up from a night's (usually disturbed) sleep. Just laying on a soft mattress for any length of time gives me back ache! But I am old, abused, overloaded, bent and otherwise, decrepid.