I have to agree that Sinclair C5's simply do not cope at all well with hills.
To make matters worse, every solution seems to create its own set of new problems.
Reducing battery weight by using Lithium Ion batteries is one option, but these are very expensive and need careful charging and maintenance. You could expect an excellent increase in range though,but no increase in actual motor performance.
24v conversion will provide a useful increase in power to help get up those hills, plus some useful increase in straight line performance. But you have to carry the two batteries to provide the 24v, increasing weight and reducing range, plus the fragile gearboxes tend to self-destruct.
Fitting gears to help with the pedalling is a good idea, but does require a little bit of engineering work.
Don't forget that getting up those hills can also cause the motor to overheat and potentially shut down to protect itself. A cooling fan is a "must".
Finally, too much effort on that rear axle can cause it to bend - it is hollow and was one of the areas Chris Crosskey had to strengthen on his LEJOG c5.
Whichever solution you choose to go for is down to personal choice. Over the winter I'm looking at a chain drive and metal gearbox solution, combined with a maxxed out 24v system - should be entertaining