Finally found the emergency boot release handle. I have owned the 159 for a year now, and my inability to spot the handle has been a nagging worry - what if the electrics failed and I needed to open the boot? This afternoon I had a more determined feel around under the front corners of the back seat, and felt something under the left side that wasn't on the right. At last I brought the loop of cable into sight, and a gentle tug opened the boot. Bingo!
With luck I'll never need it, but at least I'll be able to find it when I do.