How can I find out the number of searches per day/month conducted on
Google, Yahoo and Ask for a given term, eg flowers or shopping?
(on the .com version of each site)

There's some useful information here:

http://searchenginewatch.com/facts/article.php/2156041

Overture is the "adwords" behind Yahoo and many other search engine
results. You can look up keywords and number of times they were
searched here:
http://inventory.overture.com/d/searchinventory/suggestion/

Google's adwords doesnt seem to freely give the amount of times a word
is searched but if you sign up for the ad service you may see the
results.

This will give you a break down of the keyword and a comparison from
WORDTRACKER and OVERTURE:
http://www.digitalpoint.com/tools/suggestion/... which would be
helpful. The site also gives you the option of tracking the words over
time to gather info http://www.digitalpoint.com/tools/keywords/
although you do have to sign up (which they state is free)

The wordtracker is a pay for program that you can download.