OK, I THINK i have got to the bottom of the issue. The IT co confirmed they had entered the DHCP option 242 incorrect, with spaces where there shouldnt be spaces and missing the VLAN test 0 at the end, presumably due to trying to copy and paste it to the DHCP server at each site.. Once this was correctly the phones started coming up normally, with some requiring a hard reboot or their base number re-entering as it had been wiped out. A reboot of the phone system resulted in all phones coming straight back up..
My question is whether an incomplete DHCP string would cause this to happen, and why not all phones were affected by this? I also upgraded the software on each of the POE switches too and upgraded the system to 9.1.8, so not sure if doing any of this also affected the outcome once the DHCP option was corrected.
Hope this helps someone else,