I think building too many at a time is impossible. It is better to focus on really few (1-3) keywords first because having few on the first pages is much better than having a lot on the fifth or sixth pages.
Natural linking would not result in 100% of the links having identical anchor text. I have never seen a website sitting at #1 with all of the links having the exact same anchor text. Rather than try to prove that you don't need to vary the anchor text, it would be wiser to analyze what is already working (analyze the anchor text of the links of sites on Google's first page) and model that.
Many thanks all for your replies - site did start to climb back up a couple of days ago but has now fallen neatly back to no 31!!!!
I don't think anybody really knows how much they should be mixed up... I play it safe and mix mine up quite a bit. I like to mix in some "click here" too...
Many thanks to all who replied. I will do things different with another site!!! However - if Google has dropped my site back to 29 for "quick house sale" does that mean that it's put a flag against my site for that term and I can't now rank for it? Or is it just a case of forget about the last 2 months of work and start all over again and eventually it could improve? For other pages - with other search terms - my rankings have held good.
Check out the links from any website that is sitting at #1 in Google, for any keyword. I guarantee that they have some kind of variety of anchor text, and that all of the anchor texts are not the same. I think this is sufficient evidence that mixing the anchor text is important when building links.