Does anyone know how to bypass the temperature switch
at the source?

When disconnected from the switch, The loom end has 4 wires. The terminals are marked 1 to 5 with no connection to terminal 2.

if i bridge the switch at this point, surely this will activate the fan?

Not sure how to discover which of the connections will allow the fan
to operate.

Note: If I disconnect the switch - the fan does not operate either.