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There you have it folks... a neat Lego compatable air muscle, weighing just 2.73 grammes and capable of lifting 1kg over 17mm. I didn't test with more than 1 kg as the muscle came apart with more weight... easily repaired tho :)

The air muscle above will be just about right for J5's arms.
Currently, I envisage J5 needing 8 muscles to control his arms plus 12 smaller muscles for hand movement plus 4 even smaller for the eyelid movement, and 4 larger muscles for body movement.

I've also been researching electronic air actuators, they all seem to be a mechanical switch with a solenoid that moves the switch. Micro actuators that would be suitable for J5 cost about £70 each! J5 would need 1 actuator for each of his 28 muscles... 28 x £70 is out of reach for me!

I think i may have a solution to making cheap electronic actuators, invloving magnets, coils and more latex... a pair of actuators, one for each opposing muscle can be run from 1 RCX motor port, 1 actuator runs off the forward current and the other runs of the reverse current.
this would mean 14 RXC motor ports... 5 RCX's!

A little about the RCX motor ports...
The power and direction to each port can be set via software. When the power is set to 1 (the lowest via the lego software) the RCX still puts out the same voltage as it would on power setting 8... at a lower power setting, the voltage is pulsed... the lower the setting, the voltage pulse is shorter, and the non-voltage pulse longer.

There must be a way of using the pulse timings to control more than 2 actuators from one motor port.

A friend suggested a way of getting J5 to talk...
...my MP3 player (small + runs off 1 AA battery) hooked up to a small speaker!

I found a good cheap source of NdFeB (Neodymium) magnets...
...Neo Magnet Emporium
UK based and very good service!