immobilizer - non starter

I have just gone to start my 300tdi to go somewhere & strage goings on with the immobilizer - the red led is flashing like mad, then when I turn the key the red key symbol lights up on the dash & indicators start flashing - hence not turning over or starting - Seems lie everytime we have cold - damp - wet days I get problems, how can I bypass the full alarm - immobilier system ? the spiderbox has been bypassed with the plug in harness in the past, but im still having problems.