Should have consulted my book in the first instance. Here is what Dr Edmund Burke has to say in "Serious Cycling" Pg 97 Sore Hands
"The ulnar nerve is the major nerve in the lower arm. Descending along the inside of the forearm, it enters the hand on the palm side. Stress on this nerve causes numbness in your hand, weakness in your grip, and loss of muscular control in your fourth and fifth fingers.
Every time you ride a bike, road vibrations are transmitted through the bike to your body. Not wearing gloves or wearing worn-out gloves, using inferior bargrips, riding rough trails, and riding incorrect position can lead to ulnar neuropathy, or deadening of the ulnar nerve. Adjusting the handlebars to allow proper weight distribution, using quality handlebar grips, and wearing padded gloves help prevent or decrease pain and discomfort." He goes on to talk about gel padded gloves etc.
I know from speaking to experienced cyclists that the neck vertebrae also play an important role in pressure on this nerve as mentioned previously. Burke goes on to recommend stretching in general pre and post riding and recommends neck stretches. These posterior neck stretches as follows "
lie flat on the floor with both knees flexed while interlocking your hands on the back of your head near the crown. Exhale and pull your head off the floor and onto your chest. Keep your shoulder blades on the floor. Hold the stretch and relax.
Stretching is highly recommended for the whole body prior to riding, not just for this particular problem.