Would anyone know what spec resistance (in ohms Ω perhaps) or a part number for the tiny bead thermistor that was originally in the hole by the DVS Bus connector please? (see the photo). 

Fairly certain it's all that is needed & this olde unit will continue a bit longer... 

I've just been through the PCB & the only issue appears to be the original tint bead thermistor has completely disintegrated! 

Just a bit of connect wire remains. (Ive added a clean piece of two-core to attach a replacement to). 

I've had the PCB out & cleaned it up & by using a simple hack have proven the fan runs fine still.

The PIC controller i.c. on the board appears to be still fully functioning & the you can see data present at power up with an oscilloscope. The original thermistor appears to have simply "rotted" away. 

TIA !