Thanks Andrew
Off to the garden for 10 minutes of testing...

I'm often in manual on the camera, So I suspect it's not that, but I'll check, thanks.
It's outdoors that it gets confused. Not necessarily bright light.
One thing which will obviously not work is having it on AV in bright light, with the shutter condemned by the flash synch to slower than 160th, so over-exposure can result, but it's usually obvious when that has happened.
As an aside, I'll have a play with high speed sync and let you know what happens.

Not on mine; it's usually off.

I have exactly the same problem with K5 and Metz 58 af-1 and 2 (both updated).
I'm assuming it's me (I don't use them often in PTTL). On the K10D it was better, but A mode was always better than PTTL.
I don't think I've ever taken a successful fill-in flash shot. Skies are always washed out, defeating the object.
Surely with PTTL, nothing in the frame should be burned out? but no matter what settings I've tried on camera and flash, I just can't trust it. It's infuriating.

SMC DA 55-300mm still available. Sounds like it's important to get a good one. Something you can't check with a new one. This is a good one.
I was able to compare mine to a DA*300mm and it measures up extremely well. I literally had to check the EXIF readout to confirm which lens was used in the shot I was looking at. Both lenses wide open!

It has the potential to frame the picture, but there's not enough of it and it makes an otherwise nicely balanced picture, unbalanced. If it was my picture I would clone it out rather than cropping because I like the crop as it is.
If the shot had been taken from a slightly different angle it could have looked identical but without the tree, so I would have no ethical problem with replacing it with a bit more sky

Maybe on SDM lenses, but the screw drives are pretty robust
Anyway, have fun with it, it's a cracking lens

Shame you didn't see my 55-300 for sale here
smc DA 55-300mm ED £195

PENTAX SMC DA 55-300mm metal mount, virtually mint, immaculate box - £195 inc. P&P. This is one hell of a lens, truly stunning images in good light, fabulous bokeh and very sharp stopped down a notch.
Some images with this lens:

Except those shots appear to have been stitched by throwing the prints onto a desk and photographing them again
Lovely shot Don. Photoshop CS6 'content-aware fill' will make a fab job of the tree, which I personally would remove or show more of (if poss).