Launchd error “Could not find and/or execute program specified by service”

I am trying to get a script to run from launchd at regular intervals. The service loads but does not run the script.

The script itself is a very simple test script as follows:

echo "Hello World!" >> /Users/rhiannon/Library/Application\ Support/Script\ Testing/test.txt

The script is saved as /Users/rhiannon/Library/Application\ Support/Script\ Testing/test-launchd.sh (with permissions rwxr-xr-x) and happily runs from the command line.

I have a plist file as follows:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
    <dict>
        <key>Label</key>
        <string>uk.co.myco.timedshelltests</string>
        <key>Program</key> 
        <string>/Users/rhiannon/Library/Application\ Support/Script\ Testing/test-launchd.sh</string> 
        <key>StartInterval</key>
        <integer>60</integer>
    </dict>
</plist>

The plist file is saved as /Users/rhiannon/Library/LaunchAgents/uk.co.myco.timedshelltests.plist with permissions rw-r--r--.

I load the service using

launchctl load uk.co.myco.timedshelltests.plist

With logging set with sudo launchctl log level debug, I see the following error in the system log:

com.apple.xpc.launchd[1] (uk.co.myco.timedshelltests[13505]): Could not find and/or execute program specified by service: 2: No such file or directory: /Users/rhiannon/Library/Application\ Support/Script\ Testing/test-launchd.sh
com.apple.xpc.launchd[1] (uk.co.myco.timedshelltests[13505]): Service setup event to handle failure and will not launch until it fires.
com.apple.xpc.launchd[1] (uk.co.myco.timedshelltests[13505]): Service exited with abnormal code: 78

I'm pretty certain the permissions are correct, and I know the path is correct. What else could be wrong?