HTTP Monitor

Try this:

GET /alive HTTP/1.1\r\nHost: xyz.abc.dyx.ezt.co.uk\r\nConnection: Close\r\n\r\n

If it doesn't work, you can use this CURL command to check the output from bash on the F5:

curl -H "Host: xyz.abc.dyx.ezt.co.uk" http://x.x.x.x/alive

You can also use the following from bash for troubleshooting http monitor:

echo -e "GET /alive HTTP/1.1\r\nHost: xyz.abc.dyx.ezt.co.uk\r\nConnection: Close\r\n\r\n" | nc x.x.x.x 80

x.x.x.x is the IP of the pool member

Since you are running 11.6, you may be able to troubleshoot better using /var/log/monitors:

Look under "monitor logging" - https://support.f5.com/kb/en-us/solutions/public/12000/500/sol12531.html