Start stretching daily.That way you won't wake up in the morning feeling stiff so you can get away with training everyday which means your Karate will get better and better.

Why do you think Kawasoe can still do the splits when he's 65?He works at it daily.
As Jimmy says warming up first is so so important.Imagine putting an elastic band in the freezer then taking it out and starting to stretch it,it will snap.Imagine putting an elastic band in a oven then taking it out and starting to stretch it,it will stretch much easier and won't snap.The elastic band is like your muscles,they need to be warm before you try and stretch them.Top tip BTW is to stretch after being in the shower,your muscles are warm and loose then.
So I should try going in the oven before training?

Joke

Thanks all for the advice, makes perfect sense - will try the above suggestions, also found some good stuff on the above website - gonna start nailing some cardio in the mornings before work also - should help! I dont like the elastic band in the freezer snapping bit, makes me grimace!
Avatar is Dr Manhatten! Your telling me you havnt seen Watchmen?!
http://www.imdb.com/title/tt0409459/