Thanks Mark. This has been useful information to me.

I have installed and I'm now at the testing stage. I'm stuck on your third point:

3. You will not get ePDQ to work if the URL calling it includes the ZENID parameter eg if using a shared SSL certificate (you will probably see error "unknown order/1/r"). EPDQ needs a consistent URL to call it.
My test account does contain a phone number but I get "unknown order/1/r"

Any ideas how I can resolve it? Are you saying that I need to remove or disable to SSL cert in order for ePDQ to work?

Thanks :-)