Thank you for the compliments! Pleasure.
First, let's just be sure this isn't a ball and socket problem along with the back problem.
Lie on your back and pull the knee on the good side to your chest, and then towards the opposite shoulder. Remember what you feel. Now repeat on the naughty hip side. Is it significantly different? Where? Have your wife gently rotate the lower leg in and out (rotating in the hip). What happens?
Your saving grace is that you are so active, but you are going to have to slow down / stop for a month.
Your physio may have something. The nerve that goes to the front of the hip / groin is the Femoral nerve, and its various offshoots (like the Lateral Femoral Cutaneous nerve) which can certainly make the side of the hip / upper thigh numb. So it could be upper lumbar.
You've said several times that working in extension, arms above the head... is what seems to provoke the pain. That's good news because it points to facet impingement rather than a slipped disc.
What I would do...
* On your tum, press quite hard on the various facets, challenge them for movement, looking to see whether the spoke is the wheel is in fact at L4-L5 where you obviously have problems, or whether it is in fact higher at say L1 or L2.
* Then I would have you bend sideways, prodding and looking for the real bugger. Is it L4... or higher?
* Then I would do the Slump Test and the Femoral nerve stretch. Slump is for sciatic nerve, lower lumbar, and Fem stretch for mid and upper lumbar. Which provokes the pain?
* One thing to watch out for is weakness of the quad muscle in your thigh. If you knee starts to give, then it's more serious. Reflexes and sensory testing helps confirm all these tests.
Look up Maignes syndrome and Femoral nerve and Sciatic nerve and Meralgia paresthetica by plugging them into the Search this site function at Chiropractic Help.
Strongly suggest you start the lower back exercises at C-H (look in the navigation bar for exercises). Do them every morning before getting out of bed, and in your case several times a day.
Perhaps print this out and take to your chiro.
Good luck, let me know how you get on in a few weeks.
Dr. Barrie Lewis