To wind this thread up (and for the benefit of anyone else with CPU cooling problems in hot weather).
My extra fan blowing air into the CPU cooler duct shows:
a marginal reduction (2º) with an ambient temperature over 30ºc;
a significant reduction (5º) w.a.a.t. under 28ºc.
a probable reduction in CPU temperatures immediately after switch off (CPU fan stopped but extra fan still running).
Conclusion: if you are bored, and you've got the parts lying around in the shed/garage/bedroom, it's worth doing.