Gary there is no secret method, you just need to keep checking the hourly deals on the LITB site, pot luck really unless you sit there all day like Wes does looking for DD units to add to his collection. Even Wes only knows an hour before so you would have to monitor this site all the time. I bit the bullet and ordered one for £50 but if I can get one for under £10 I'll get one or two, always handy to have a spare
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Still well chuffed with my 6cm unit from the other thread, working great.
Blackrose