@terziyski 

Thanks for your reply. You'll have to forgive me as it's been a while since I last changed anything with the network and the brain's getting a little foggy. To get the picture straight in my mind, I keep the modem connected to the old router through the WAN port and then connect the new TP-Link to one of the 4 ports on the old router and disable the wifi in the old router, right so far? (you said wireless radio, I'm not familiar with that term). What about DHCP? I'm thinking I also need to turn that off in the old router and turn it on in the TP-Link? In my set up I'm using two old routers as AP's at either end of the house, is it Ok for the new TP-link to be set up as an AP and connect (by wire) to other AP's? I will also have an unmanaged switch connected to the TP-link, all of that is possible?

If it's too complicated I'll just do away with the DECT phones as we don't really use the landline much any-more anyway.

  @terziyski Ah yes, the fog is clearing. I leave the original router as the DHCP server and the old routers and the new AX23 set up as AP's with DHCP turned off, got it. I can't test the AX23 yet as I'm having trouble getting an internet connection from the modem but that's a separate issue and probably deserves it's own thread but I've found a few things on the web about setting the Internet VLAN in the AX23 to either 1074 or 20 (Jazztel in Spain) and I'll try those tomorrow, no time today. I did try it yesterday but it insisted on having an entry between 2 and 4094 in the VoIP VLAN ID and I wasn't sure what to put in there (have since seen that again it should be 1074 or 20 but couldn't get everyone off the internet to test it). I'll let you know Sunday/Monday if I've got it all working, 

Thanks again for your help, much appreciated. 

  @Hotblack It turns out the videos I saw where they were entering 1) the same info into VoIP VLAN and Internet VLAN and 2) only entering info into VLAN were both wrong. After lots of trial and error I entered 20 into Internet VLAN and 2 into VoIP VLAN (it had to have a number between 2 and 4094 and it couldn't be the same as the Internet VLAN) and that eventually worked. At one point I hit the 'clone MAC address' button as I was running out of ideas but I can't say for sure if this was necessary to make it work but I'll mention it here in case anyone else is going round in circles, it might be the missing piece. Anyway, I got it working, I had to change all the PCs that had static IP's back to Obtain an IP address automatically to get them to connect to the new router. Not sure why it didn't like the fact they had a static IP but hopefully I can switch them back at some point. 

I couldn't get anywhere with Jazztel with regard to plugging the phone cable into the modem. They seemed completely clueless as to what I was talking about and didn't call back despite saying they would so for now, no landline.