Can a small 12 volt 200 watt electric motor run safely off a deep cycle battery whilst the battery is being charged at say at 20 amperes?

Should be OK.

If the motor draw is 200 watts, and the charge current is 20 amps, you've got (12 x 20) = 240 watts in and only 200 out.

The battery will charge at a rate of (40/12) about 3.3 amps, so be aware that

(i) the battery will take longer to charge than if the motor wasn't running, and

(ii) once you stop the motor, the charge current is the whole 20 amps and you'd need to be sure that the battery won't overcharge.