The middle layer was mainly the John Innes seed compost, because that's really sandy and I read that carrots like sandy soil. I mixed a bit of the B&Q Value stuff in with it too because the John Innes was a bit too heavy, and the B&Q Value helped with that.
The top layer was a fairly equal mix of B&Q Value with Miracle-Gro potting mix, because they're both really light and my other plants seem to find it a bit easier to sprout through the finer soils.
I put quite a few more seeds in there than I should have, but I'm going to pull up the weakest ones when they start growing, so it won't be a problem.
After that, I tried to fill in most of the other gaps with radish seeds. They're super easy to plant too, I just roughed up the soil a bit, sprinkled seeds over it and then smoothed the soil out again so they were covered.
The reason I planted radishes in with the carrots and parsnips is that I read they grow nicely together. The radishes fill in the empty spaces and finish much quicker than the carrots, so when you pull them up it loosens the soil a bit so the carrots and parsnips can grow better.
Lastly, I gave them a nice big drink of water so they'd be moist enough to germinate.
I'm quite happy with how it turned out, hopefully I'm rewarded with a tasty harvest in a few weeks.