Can I strongly recommend that you try and get some teaching experience before doing the HDip - as already suggested below. 1.To see if you actually like it (been there and discovered I didn't), 2. Helps to increase your points.
It is possible to get substitute work - it's well paid. I'd go in to the school first thing every morning on the off chance that a sub would be required. I typically ended up with 3-4 classes. Think about extra curricular activities that you can help out with - this was one of the ways that I got to do a fair bit of teaching. I realise if you're working - it's a big commitment to give it up, but wouldn't it be worse to go and do the HDIp and discover then you don't like it!
Like others have said, it's difficult - but not impossible to get in. It's a points scale and interview if I remember correctly. Look up other sites and I'm sure you'll find more information on this and do what you can to increase your points.