am trying to take out old washing machine, but am unable to turn of stop valves on water pipes as they appear to be in a fixed position with a small screw in top- how do i turn them of? do i have to turn water of at stop cock and will this affect my combi boiler?
Washer shutoff valves typically get corroded and won't budge years later when you try to turn them. Althought the handles held on by the screw may crumble, you can try gripping them with channel-locks or a pipe wrench to turn thenm clockwise to close them.
If that doesn't shut off the water, you may just have to turn off your main.