After 2 months I finally got it, I'd become fixated on a particular route which is wrong and then on nearly the correct route but kept going round a block which needed pushing out of the way. Fine picture by the way.
I found only one area where careful routing was needed, the rest being relatively straight forward. However it did take me weeks to find the correct path so not particularly easy IMO.
It took a couple of attempts to realise I was starting on the wrong side, then a couple to reach the exit and one more to get the 40 steps. Not completely obvious or simple, a nice challenge.