I have a site that ranks nowhere (or occasionally 300-400). However,
it is in the top 20 when I use any of the allin commands and has done
for 3 months or so. Can anyone shed any light on this ?

When you say "allin" I assume you mean 
allintitle
://www.google.com/help/operators.html#allintitle
and allinurl
://www.google.com/help/operators.html#allinurl

That your site is highly ranked when using these restrictions just
goes to show that you have made a website with much more descriptive
information in the
<TITLE></TITLE> tags.  Many sites do a poor job of the titles and do
not put useful information in there and instead repeat the site or
company name over and over again.

Many sites including google display complicated URLs, such as the
location of this page
http://answers.google.com/answers/threadview?id=391761#a
where as other sites provide more user friendly descriptive URLs

if google wanted this page to be found more easily they might setup
their website to rewrite the location as something more user friendly
http://answers.google.com/answers/meaning_of_allin_results/

I can only assume that your site has done a much better job in this
area than most sites but that the rest of the content on your site is
not as well tailored to get a high google ranking.