I’m finally getting to grips with WordPress, it’s that “click” moment I always (eventually) get with most things.
The final puzzle has been solved, in no small thanks to Stuart of foolfillment fame who left a link to a WordPress plugin in a recent comment. The plugin allows you to have a different number of posts displayed here on the ‘home’ page, on the search page, and in the various levels of archive page.
If you do want to use it, just edit it with a text editor, setting the limits as you desire, then upload it to the WordPress plugin directory, activate it et voilà! Go on, try it out. Search for something, or head into the archives. The search will return 25 posts, the archives will show 50 posts for each month and ALL the posts if you choose a year (might be advisable NOT to do that). The main page (that you are reading) is currently set to show the last 10 posts, but that can be changed if it’s causing the page to load slowly.
I’m still bemused as to why this isn’t standard functionality mind you.