Can I prove the rotor is free or stuck without removing the motor?
Yes. Unplug the machine and remove all its filters. In the sump, to the right there will be a hole, this is the inlet for the circulation pump. It may be covered from above by a crescent shaped plastic piece which will just pull out easily.
Being very careful, you can rotate the impeller with your fingers. If it wobbles up/down/left/right, then it's likely that your bearings are dodgy. You could also remove the right side panel and turn the rotor from the back of the motor.
Can I check the capacitor if it is phase shift - without buying a new one?
Question is how do I get the motor out - or the capacitor - do I need to remove the base - if so how?
You can access the motor by removing the right side panel. If you actually want to remove the it, it may be possible to remove it without removing either tub or base, but you'll have to be very careful, but I haven't tried this. You could remove the tub, but again, this comes with caveats, particularly since the metal may deform easily.
This might help you
http://www.applianceblog.com/mainforums/bosch/1930-removing-base-access-pump-bosch-dishwasher.html
What is the little black clip on - on top of the motor with two spades to the motor connections and a third wire off?
Unsure, can you post a photo?
On a separate note I have checked the heating element and this is fine, however there is what appears to be an over heat sensor by the heater terminal but this appears open circuit. Is it an overheat sensor or something else?
This will be a temperature sensor, not just overheat, so that the dishwasher can adjust the heater as necessary to control the temperature. As far as I am aware anyway. If you do some exploring on bosch's website, you can find entire diagrams detailing spare parts and their descriptions.
Find your Enr number and enter it
here.
Hope this helps
~easkay