Desktop Display Locked on VNC Client accessing Centos 7 Compute Engine

I have a centos 7 compute engine. It has vncserver installed. I have started vncserver :1 and can check it is running:

$ vncserver -list
TigerVNC server sessions:
X DISPLAY #     PROCESS ID
:1              26094

I can check the local host:

$ nc localhost 5901
RFB 003.008

$ nc localhost 22
SSH-2.0-OpenSSH_6.6.1

I am using a Windows7 PC to connect and the TigerVNC client version 6.0.1 for Windows-64bit

When I run it, it prompts for a password. It accepts this and on the log I see:

Fri Nov 25 16:29:54 2016  vncext:      VNC extension running!  vncext: Listening for VNC connections on all interface(s), port 5901  vncext:  created VNC server for screen 0 xterm: cannot load font '-misc-fixed-medium-r-semicondensed--13-120-75-75-c-60-iso10646-1' gnome-session-is-accelerated: llvmpipe detected.

Fri Nov 25 16:30:27 2016  Connections: accepted: 85.237.223.111::63460 SConnection: Client needs protocol version 3.8

Fri Nov 25 16:30:28 2016  SConnection: Client requests security type VncAuth(2)

Fri Nov 25 16:30:44 2016  VNCSConnST:  Server default pixel format depth 24 (32bpp) little-endian rgb888  VNCSConnST:  Client pixel format depth 24 (32bpp) little-endian rgb888

On the client, I enter & it accepts the password and connects, as I have previously set up the firewall rules for vnc-server, including a label on the server.

However, all it displays is a coloured screen with a pattern on it of tiles in various shades of green, and a digital clock display in the centre showing the 24 hour clock and the day and date, as if the screen is locked and I can't unlock it to get to the desktop.

How do I resolve this?