Still no further forward with this due to lack of time but my monster home DIY project finsished yesterday woohoo so can get under the bonnet (or dash).
From reading back through dozens of theads with similar (but nor always exact) problems it sounds like I have to replace the remote ECU and either get my fobs recoded by Volvo or find a breaker with the same unit and fobs;
https://www.volvoforums.org.uk/showt...=remote&page=3
https://www.volvoforums.org.uk/showt...ghlight=remote
What a ball ache. I don't know if it makes any difference to the prognosis but every now and again when walking past my car on the drive I give the fob buttons a little press just to see if the Volvo pixies have magically fixed it. Two or three times when later getting back in the car I have found this casual button flip has been enough to set the alarm, but not the flashing red light on the dash. Opening the door sets off the alarm which has to be silenced by the key in the ignition.
One of the reasons I hang on to my Volvo is the lack of electrical gubbins, goodness knows what horrors I have to come when eventually updating to something from this century.