[UK-CONTEST] Alpha fans

[G3RAU at aol.com]

On the back of  some Alpha linears it's possible to bolt on  an auxiliary 
axial fan
 to keep everything running nice and cool during contests.  
First question is, is this a 38 or 25mm thick fan? 
 I presume when the auxiliary fan has been fitted the  airflow out of the 
chimneys
 increases to an extent that is detectable by hand ( and ear?) - but  
roughly
 how much increase is there?
Second question is, have you used a UK sourced fan that does the job,
 rather than the factory supplied one,  and is it  quiet?
Interested in peoples experiences before buying  something.  
TIA
Derek G3RAU