When you signed up and created your site, the hello world post was created by the wordpress software automatically so that you would not see a 404 error message on the main page of your site. At that moment, it showed up in your RSS feed. You then probably deleted it at posts > all posts in your dashboard, but as raincoaster says, it was already in your feed.
When I look at your feed at http://halifaxcharityknitters.wordpress.com/feed/ I do not see the hello world post. Where are you looking to see it?
Also, with your posts being HEAVY with images, I would suggest that you set your feed to "summary at settings > reading.
And due to the fact that your theme uses infinite scrolling, I would also use the "read more" tag in any post you create so that most of the images are behind the read more tag. I waited 5 minutes for your site main page to load and then cancelled out and it was only about half way loaded.
This is one of the main reasons (image heavy posts like yours) I argued for a way to turn off infinite scrolling because for people on marginal or slow internet connections, it drives people away. They simply will not wait for everything to load. Before infinite scrolling hit your theme, you could set the number of posts that you wanted to show on your site main page, but you cannot now.
Thanks Matt and AutoMATTic.