FRITZ!Box 7530 as DECT base
FIXED
OK, as previously mentioned I managed to register the FritzBox and get the little green light for the connection using (mainly) the VOIP provider's credentials. I did set up a fixed IP address on the PN Hub Two for the FritzBox, which I'm not sure if it made any difference but decided to leave well alone having done it. No special porting arrangements needed.
So both phones (handset to DECT, SIP phone to LAN) were ringing, but not at the same time. I then bought two Gigaset handsets from eBay as a replacement for the DECT, which were very straightforward to register with the FritzBox as base station. They also have the advantage that they can be used with the FritzBox Eco Dect facility. They rang together but were still either/or versus the Yealink SIP phone. But any of the phones could dial out, and even while a call was in progress on the other connection.
The secret of getting them to act as one unit was to then change the settings in the SIP phone so that the FritzBox was the registrar and NOT the VOIP provider. So all then rang successfully together at last! And any phone could answer or dial out.
This is all on one VOIP extension and I presume that the FritzBox represents the extension so you could have more phones connected than I have.
The last fly in the ointment was the voicemail. This was going to the VOIP provider's system, and although I was getting an email with attached message I couldn't use the provider's dial code to access messages – this just resulted in a 'call failed, busy here' error. Missed calls also set the FritzBox Fon/DECT LED flashing continuously and unable to be switched off (it always thought that at least one phone remained unanswered despite deleting all missed calls on all phones?) So I also then felt forced to set up the FritzBox answering machine but couldn't get this to override the system until I realised it was just a matter of shortening the answer time so that the FritzBox system gets in there first! This works well now using the FritzBox internal code on any phone, includes email push notifications, and presumably in case of a power cut or other problem will revert to the old VOIP system anyway as a backup.
Because the phones are now all networked through the Fritzbox there is the added advantage that any of them can be dialled from any other, so future proofed in case I need a day in bed and want something from downstairs! 
