Hi Peter this sounds like the valve or actuator has failed in the washing machine, and not relating to the waste water (outlet) pipe.
If by switching off the stop cock the washing machine doesnt fill up this most certainly proves the fault is with the valve.
The good news is a replacement valve is around £15 from here.
My suggestion is turn off the stop cock if the machine doesnt fill up, arrange for an engineer to come out and replace the valve, this should be a relatively low cost fix, however if the machine fills up with the stop cock off, then this would suggest a drainage (outlet issue), and you need to check if there is a problem with how the water is flowing away on the drain.
Please let me know if you require any further advice or me to clarify anything raised here.
Thank you for letting me know, I would be surprised if you mains water supply was above that of the tolerance for the washing machine, however appreciate letting me know.
If your confident the pressure is above the washer spec, I wouldnt worry, however if it isnt and simply reducing the pressured fixed the issue, would suggest a o-ring failure in the valve, which will only get worse.