Yes very easy to replace, and fairly cheap. Around £6.
I'm in the process of doing it right now. Made an account here just so I could reply to you.

Just go on ebay and search "ford ka seat knobs".

Or buy one from here: http://www.fordpartsuk.com/shop/ford...6116_c_123.htm


And then you just push it onto the metal lever that should still be the in the seat if you've only snapped the plastic.