@donalcrowley  thank you for your reply, do you know is there any other extender I can add - for example the TP-Link TL-PA7017 can this be added to my TL-WPA8631P Kit  ...or does it have to be another TL-WPA8631P 

donalcrowley wrote

I have TL-WPA8631P Kit (AV1300 Powerline) - can I add additional Extenders (which ones should I get?) 

  @donalcrowley I do not know the answer to your question. However, for the additional extension do you want a powerline extender only or do you want a powerline extender with wifi. If you want an additional powerline extender with wifi then I think you need the TL- WPA8631P.

  @David-TP 

thanks for the link - but it doesn't answer my question. I understand it is possible to add additional extenders (and only require one adapter that is connected to the router), but I'm trying to find out if the additional extender has to be the same as the one in my kit i.e. TL-WPA8631P.

Previous reply from StCo said it's not possible to buy the TL-WPA8631P on its own (I could by another kit and throw away the adapter TL-PA8010P and just use the extender TL-WPA8631P ...but at €120 for the kit this seems expensive way to add just one additional extender) - so I'm wondering can I add any other model of TP-Link extender such as TL-PA7017 (which costs €55) ?