Hi Dominic,
As Chris has mentioned your pressures don't look right. You yourself have mentioned the key/drive shaft on the lift pump, have you checked this yet? It only takes a few mins to do.
Regards,
Bryan.
PS, Forgot to say, in relation to your question about voltage, that as Chris and Mark say, this is probably PWM, so you wont get the same volts as a supply or earth. It will vary according to the duty cycle. HTH's