Thinking about it, the various schemes suggested by others for having DECT phones connected to a mobile via Bluetooth are rather clever, but I don't see how it would give me any big advantage over my current arrangement. I'd also need a new mobile as the current one doesn't have Bluetooth. I wouldn't save on line rental as I still need the line for ADSL and the call costs to and from mobiles are higher. A better plan which I have been considering is to go over entirely to VOIP. My ISP will sell me a VOIP service and I already have Asterisk running on my server and connected to my DECT base station which also does SIP. Call costs would be lower than with the landline but the user experience with the DECT phones would stay the same (very important for my better half).
I could even take advantage of my ISPs SIP2SIM service to get a SIM card which links directly to the same system.