Thank you for your response Basara.
I had it set as per their requirements as mydomain.co.za, so I have now tried it with www. ... and it's still not working.
(reCaptcha does not accept https:// ... I'm sure you already know that, but I tried it anyway)
I must say I really hoped that, that was the solution, even if I would kick myself for it!
It probably is something very simple, I just can't fathom what it is. I deleted the first set of keys Google reCaptcha gave and tried again with a new set, making sure that I followed the instructions exactly as per the AbanteCart user manual and Google reCaptcha settings. As far as I understand and according to what I read on another post on this forum I am not required to do anything on the Client or Server side integration as AbanteCart does this for me..or have I got that wrong? And could this be a step I missed?
Thank you again for your time.