Waldorf - not checked it in detail myself - but perfectly possible that the available bid/offer price was lower but no trade occurred at that level?
Edit: Cannot now get advfn's quote-changes analysis, but with the shaded spread on advfn's intraday chart seems to show a bid price of 173.5p existed between (roughly) about 2:33pm and 2:55pm (and 172p briefly before that).
Not sure about that, would like to know why though it didn't get triggered earlier. I noticed you shorted at 266, when did you actually set the stop?.. presumably the price went lower than that, you set the stop and it went back up later.
Looked at ADVFN and 2:45 to 2:55 it wasn't 267, so we can discount that, looks like it was 267 for about 7 mins like you said - that should have triggered it, so can't say why it didn't - but I think we can blame the delayed feed. Hopefully some day we can upgrade to a proper realtime feed, which should solve these inconsisitencies.
In the meantime I'll update your portfolio with the correct price.
ps. Not so far off the top now,and you're already 2nd place for this week
I can't retrieve yesterday's data from advfn now, but my own scribbled notes show the market price 266-266.5p when I set the 267p stop, which would have been at or immediately following the original shorting order. I don't think(?) I changed it, but can't remember well enough to be certain.