I think you are likely to get more feedback about this in the "Open Forum" .

I was thinking of a Legacy 40 for my next project.  I have the plan and it seems like a big airframe with a very thick wing section  for a 40 sized model.  Power would be an OS 46 LA or perhaps a reworked ST 51  Has anyone any experience of this model?   I have a 60 sized Legacy and it flies well - just wonder about this 40 size  as I can find very little in the way of comment on it.

Thanks

That's because there just isn't that many Legacy 40 models flying. The original Legacy is much more popular (I'm building one myself).
It should fly great with the .46. The only one I personally know of uses a Brodak 40 and it flies very well at (I think) 49 oz. Seemed heavy to me but still flies good. I would think the ST 51 might be a little overkill but would still work if you set it up right. The OS46 is lighter.