Sunday supplement critics frequently bandy about such phrases as "long-awaited conclusion of the trilogy", but in this case - well, let's just say that the second instalment came out the year I was born, and Part One 52 years ago, which means that the trilogy covers exactly two thirds of Garner's own life. (I'm quite happy to suspect him of numerological shenanigans in this matter.)
Meanwhile, his characters (or at least Colin, the only one left in our mundane realm) have grown up, and out, of the earlier books' world. Gowther Mossock is presumably long since crowned with a tussock, the Morrigan long gone. Colin is now a Professor, and Garner's near neighbour at Jodrell Bank observatory. (Will we have a postmodern cameo from the eccentric novelist over the way? I rather hope not.)
Several years ago, when this book was in its early stages of gestation, I suppose, Garner hinted to me that the next work might have something to do with Ludchurch, and the Gawain chapel. Is that still the case? Who knows? If it is, it won't be in any simple way.