After many weeks of head scratching, measuring, and just plain old trial and error, I've finally managed to get my Wye into place on my Beccaton, Michaelsberg and Williamstown (BM&W) loft layout.
It's not as long as I'd like it to be, the curves are down to 18" on two sides, I can't get the 6 axle locos round it (and even one of the 4 axle ones won't go round), but it's in place.
Having laid the track I asked Brain Woolven to help me test it. With his help we determined that one curve needed easing to get the locos to go round. This was easily resolved by simply loosening the screws on the 'screw and washer' method I use to hold track in place while I complete wiring, fit point motors, and generally test before fixing with glue at a later date.
Thanks to Brian's help I was able to test most of the 4 axle locos which went through all sides of the Wye without too much difficulty. Photos of parts of these tests can be seen at.