There is no way currently to force your photos together if they don't synth, although the Photosynth team has heard us loud and clear and promised that they are working on making that feature for us.
I took a quick look at your Coombe Hill synth ( http://photosynth.net/view.aspx?cid=11cd6220-9130-4d5f-a78b-0317679043e7 ) and saw a few places that I was surprised to see photos not synth together like when the tall monument is in the far distance (although higher resolution photos would help you). I also saw quite a few places where you should have taken some more photos when circling monuments.
When taking photos in a circle around an object 36 photos is usually guaranteed to get the photos from all sides to tie together, although I'm not saying you NEED that many. (I wouldn't take fewer than 20, though.)
Lastly, if you're taking groups of photos at different distances, make sure things don't get more than twice as big in between those groups