Suzanne Bell wrote:

when I isolate the water in the flat via stopcock in cupboard I dont
get any water from hot or cold tap but water is still gushing out of
pipe at washing machine if I try to disconnect

Just what's left in the pipework, or gushing out at mains pressure, get
a bucket and few towels and find out of it stops ... if it does you can
cap-off the faulty valve, e.g.

https://www.screwfix.com/p/bsp-blank-nut-x/39553