All good and useful info. My problem with this model was that it couldn't be done using booleans. Because of the smooth transition from the curved neck into the flat indentation it had to be made from blended surfaces.
As Michael pointed out, the best method in the end was to break the area down into individual sections and blend them, then sweep the last bit where the profiles came together. Worked like a charm and looks lovely. As this model will be used for prototyping and not rendering it had to be solid. I did a purely sub-d version for the renders as accuracy wasn't necessary and uv-ing for texture maps was easier.

Back in the olden days, (maybe 15 yrs ago now) there used to be a modeller that used a kind of 3d version of bezier splines like illustrator. I think it was Electric Image or early Shade or somesuch. I remember you could edit the splines in 3d just like illustrator and it would constantly update a surface between them. It was very clever. So for a model like this you could literally draw the outline of the indent on the bottle surface and pull it down into it, creating the depth, all the time retaining the editing ability of the splines. No idea what happened to it though. That was back on a mac classic and long before iMac's etc.