It is now nearly 1800z on the 24th and the possibilities are narrowing - either SDG-9 is down, or it is flying at around 8km.
The latter is entirely possible and fits well with the temperature data that was received by WB2YDS early in the flight. This lower altitude also greatly increases the chance of frost downing the balloon, but it could still be flying. Were it at the design altitude of 9-10km, then it should already be over Spain at this point (extremely unlikely, unless the transmitter has somehow failed).
If it is still flying and at 8km, then it should be over Greenland right now and about 12 hours away from the UK. Keep listening, folks - it isn't over quite yet!
73 and thanks to all for your help,