Whilst the performance of the Heljan Class 47 is normally exemplary, I've had a handful of lame ducks sitting around awaiting attention for well over a year. These locos would run jerkily, switch their directional lights at random and even change direction of their own accord. With more than enough 47s to cover services on my layout, tackling this has never been a high priority, but now I've solved the problem I wish I'd done it sooner.
My erratic runners were merely down to the pins & solder on the underside of the circuit board shorting out against the metal chassis. Slap a piece of insulating tape over the offending areas and the problem is solved in less than 2 minutes. The black tape in the inset photograph is my 'fix' and the smaller piece of yellow tape is original Heljan. Thanks are due to Richard Pedder for giving me a clue on this one at the appropriate time.