Extended Desktop Issues - 2nd monitor "No Signal"

I have just been given a 2nd monitor. It has only a VGA connection. My existing monitor has both VGA and DVI but the DVI port has been non functional for some time.

My graphics card is an ATI Radeon HD 5700 Series which has 2 DVI ports but no VGA.

I was using my old monitor attached VGA cable to DVI port using an adaptor (DVI M, VGA F) to attached to the graphics card which worked fine.

I have replaced this with the new monitor, which works great with that setup.

The old monitor I have set up using a DVI cable to VGA port attached using an adaptor (DVI F, VGA M) to attach to the monitor. The monitor is detected by Windows (and AMD Vision CC) but only displays No Signal. I have checked the refresh rates and screen resolution and they are fine. Drivers are up to date. There is nothing in the BIOS that should affect the display. I have had dual display with extended desktop working in the past with an LCD TV as the 2nd monitor (using HDMI). I have 2 DVI cables, the problems exists with both.

This confuses me. What is different about a VGA cable + adaptor compared to a DVI cable + adaptor setup?

To get this to work do I need to:

1. Tweak some setting? If so can anyone suggest what?
2. Buy a new VGA cable + adaptor?
3. Buy a VGA to DVI cable?

Many thanks in advance.