How to demonstrate typing ability?

Just put on your CV that you're "quick and accurate", if you need to put a figure on it then put a number. I wouldn't worry about certificates, just make sure that if they decide to test it at interview (one did for me last year, I hadn't claimed anything, but they tested typing speed) you can match the level you claim to be at.

So if I just do one of the typing tests online and put my score in it'll be ok? I'm just worried that won't hold much weight. Thanks for your answer.

I'd do a few different tests to make sure that you can match your claim, and then just put it into your CV. My speed is 70 comfortably, up to 85 if I'm familiar with the keyboard I'm using and the room isn't cold. So I'd put 70 on my CV if applying for jobs where a high typing speed was necessary.