Hi all, thought I would add something as someone who spent most of my childhood in the nearby Pitshanger Park and I know that area extremely well, I live about a 5 minute walk from the crime scene. I even went to Gurnell leisure centre before it closed each week for about 5 years as a child, but I found the appeal of the witness of a man as "if he had just washed, walking across the footbridge that crosses the A40 Western Avenue, Greenford, on the day Penny was killed." particularly interesting.
In my 20 years of going around that park I have never seen significant activity in the adjacent green that coincidentally goes through an isolated section of the river Brent, unless it was for football matches on the weekends. The odd dog walker yes, but people using that green, especially before the construction of the skate park on a Thursday during the day would have been almost nobody. Most people would visited Gurnell car park for the leisure centre and its facilities, the rest would go to the more popular Pitshanger park next door or the other park located on the other side of the footbridge. IMO the killer could have spent as long as he wanted washing himself in the river and taking his clothes off. One section even goes under a bridge in which numerous cars pass. Unless you are specifically looking over a 15 foot drop there is no way anybody would have spotted him. that green leads directly to the bridge via one road with multiple houses, but running you would pass the houses within about 30 seconds and at the end it goes to exactly the bridge where the witness saw the man . On the other side is a collection of houses, perhaps his car or home was here? Not sure if this is of any use but thought it might be.