Well my attenuation has returned to "normal" i.e. 37.5dBm and it's still zero outside or thereabouts so there's always one example that bucks the theory.
Since moving the router from the unheated room to a floor location in a bedroom I've had no reboots in 24 hours.
Current stats are :
Uptime: 1 day, 0:04:38
DSL Type: G.992.5 annex A
Bandwidth (Up/Down) [kbps/kbps]: 1,325 / 8,187
Output Power (Up/Down) [dBm]: 12.0 / 19.0
Line Attenuation (Up/Down) [dB]: 19.0 / 37.5
SN Margin (Up/Down) [dB]: 7.0 / 6.5
Vendor ID (Local/Remote): TMMB / BDCM
Loss of Framing (Local/Remote): 0 / 0
Loss of Signal (Local/Remote): 0 / 0
Loss of Power (Local/Remote): 0 / 0
Loss of Link (Remote): 0
Error Seconds (Local/Remote): 0 / 0
What is strange is that even though the router is now using a hard-wired extension box (ring wire removed) I am seeing no Loss of Signal or Error Seconds which I was seeing using the main BT linebox even when the room temperature was 18c+.
There are FEC, CRC and HEC errors but nothing to concern me for a 24 hour period.