Thanks all for the replies!
I guess what I'm really after is added range. I need a lens I can use at events where there's more distance between myself and my subject than can be overcome by my 50mm and foot zoom
I had a Tamron 28-75 for a while and felt that 75mm didn't give me that much more than my 50mm. So the DA70 and FA77 are now logically out of the running.
I also had a Vivitar 105mm macro. Loved it and would have kept it but for the lack of AF. I didn't use it much for macro though, and on hindsight that was probably due to my lack of experience- often used it handheld and wide open so I could get fast enough shutter speeds, consequently getting razor-thin depth of field.
The Sigma 100-300 was an interesting suggestion, but it's just out of my price range, even used. As is the 60-250.
A question about Sigma HSM lenses, specifically the 50-150 or 70-200- should the HSM fail, is there a way to use them with screwdrive autofocus? I intend to keep this next lens for life.