We eventually found the apartment after a short while driving around residential streets and knocking on unsuspecting Croatian families doors !! Although it isn't a large property we found it very comfortable and located just outside the main bustle of Opatija. Didn't really speak to the owners apart from hello and goodbye but I don't think they speak English too well.
I would definitely recommend this place as the price was fantastic and there's also a great restaurant about 50 metres away..Zlatni which I've also reviewed.
If you do stay here, I hope you have as great a time in Istria that we did ☺ more