Hi Suzie,
Yep, fixtheodds.com. If you're going to try it, I would strongly recommend that you read all the instructions to the bone the first time around as it does get (much) easier the second time round!!
Few tips...
On the 'setting up bet', where you make a small loss, it makes a suggestion saying.. you place this much and you will lose this much... and there's an option to say whether you're happy or not with their suggestion. I would recommend that you choose 'not happy', as it will make another recommendation which may make a huge difference.
For example, on the second time around, they suggested that I would make a loss of £2.10. However when I clicked 'no i'm not happy', I ended up with a suggestion which meant that I would only lose £1.06!!
I'm not sure why they don't automatically choose the lowest loss, but it is worth going through their suggestions.
Also top tip.. there's a screen where it says... you will need £xx.xx in Betfair and £xx.xx with some bookmaker...
However, it isn't actually clear that at that point that you need to deposit the recommended amount in to Betfair. So for example, if it says you will need £20 in Betfair and you only have £10, you need to deposit £10.
Personally and as Potato suggests, I would put a larger 'float' - so for example if it says you need £20 and you only have £10, I would put £11.
This just in case that if the odds change and you require a penny more, you don't end up having to deposit again!!