Rainer wrote 

I have a 91 westy with 75K miles that has just developed a coolant leak
(trickle) emanating from the left side cylinder head at the upper cap bolt.
Any suggestions?

Most likely the head is cracked if its leaking from the head bolt area. If you clean it you will see that it is leaking at the raduis of the bolt seat surface. I sugest you  do both of the heads at the same time and be done with it 

Bob 

http://www.bostonengine.com