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The optimum solution to Fiendish can be found here.
Fiendish was devised by hand, over a period of several days. My sole aim was to improve on my previous best (Route66) which I had stumbled on more by luck than judgement, and was a huge leap forward in its time. A nifty windowsbased (C++) plankpuzzle editor/solver (by applet author  Graham Rogers) was used to verify the solution. Fiendish is actually shorter than Route66, but more complex and compact, as (hopefully) illustrated in the following table. Note I (used to*) measure plank puzzle complexity by moveoverspan ratio which is why I rank Fiendish above Route66. Fiendish and Route66 are the only puzzles to date (03/03/01*) with a moveoverspan ratio greater than 1 (meaning the number of moves actually exceeds the number of possible plank positions).
Route66  Fiendish  
Grid  9x9 (square)  8x8 (square) 
Planks  3 (1+2+3)  4 (1+2+3+4) 
Shortest solution  66 moves  57 moves 
Span count  54  40 
Move/span ratio  1.22  1.42 
(*) In late March 2001 Pascal Wassong well and truely smashed my moveoverspan ratio record with a plankpuzzle of his own invention entitled The Centrifugal Force. This puzzle requires an incredible 342 moves and has a moveoverspan ratio of 4.62. Try it here.