Well having gathered together everything I needed I did a bench test of the set up today and everything ran quite well on 3.7 volts.
So much so that i couldn't resist securing the motor into the hull (wonderfull stuff is marine silicone) and just placing the rest in. The little beast runs at quite a nice speed and is much quieter on 3.7 volts. I tried taking a few shots in the test tank. Not very good ones with the camera in one hand, the radio in the other. Cross fingers, see if I can load them up OK.