Flippin' 'eck, this should have come with a health warning! Despite a lucky guess at the entry method of the down clues this was a slog of at least 5 hours before the grid was filled, and it took a further 2 hours to get the last of the cryptic indications of the secret sign even though I knew what I was looking for. It didn't help that this wasn't in my usual reference source.
I only stuck it to the end because having got so close it seemed a shame to give up, but found the whole thing frustrating and I can't say I enjoyed it. BUT I'm not blaming the setter for that - Listeners are supposed to be difficult and the series has to cater for super-solvers (of which I am definitely not one) with the occasional stinker. Usually I can spot these and avoid them (they usually have blank grids, coded answers or are headed by the name Sabre). This one didn't look too bad at first...
I can appreciate that this was a marvellous feat of construction though I agree with Scorpius that some of the clueing could have been tighter, given the obstacles we had to surmount. As I get more advanced in years I'm not sure I can, or want to, justify spending this long on a single crossword very often, but some people clearly enjoyed it more than I did and so it would be churlish to complain about its inclusion.
Jago or Dipper next week please!