OK, thanks. My understanding is that AIS is point-to-point so, as has been said,it must have been atmospherics combined with strong output. I was unable to receive ship's information just location but suspect it was one with VHF transmitter on tall mast and high powered. That being said, some large tankers close by showed up at the last minute. Anyone else had fun crossing the 5-lane TSS off Cape St Vincent in frisky conditions? 
There are several articles describing unusual propagation of VHF radio waves, one of which is at the following site
http://dxfm.com/content/propagation.htmOne of the points the article raises is that while VHF transmissions tend to be considered as line of site, it's actually a bit more than that, and considerably more if atmospheric conditions happen to be just right. In one article it reported transmissions exceeding 3700 kilometres. The extremely long ranges I experienced had been with standard equipment as provided on an ordinary merchant vessel where the radio systems provided onboard were standard off the shelf units, and not high powered. Indeed increased power would not have made any significant difference to the ranges achieved where this was due solely to the atmospheric conditions. As for the height of the ship's antenna, yes they would normally be placed as high up as possible in order to achieve maximum range.
I can't comment on the tankers that showed up at the last moment, perhaps something was shielding the transmissions.
