You won't generally get good root crops from new beds, as if done properly you'll have dug in manure or some other soil improver. Carrots, parsnips etc have tap roots that go down to find nutrients, if your soil is too rich, then they won't grow downwards to find those nutrients - it's why they do well in fine, sandy soil, they are looking for 'food'.
Personally, I'd use the old compost you have, add a slow-release pelleted fertiliser (either chemical or pelleted chicken poo if you want to be organic), but I'd mix the compost with sharp sand - it's cheaper than horticultural grit, and you can buy it from a builder's merchant. I'd use about two-thirds compost to one-third sand.
Hopefully you'll get better results this year anyway. Can I ask why you've decided to go with one mega-huge raised bed, instead of several smaller ones where you can practise crop rotation? I know you can still rotate crops in the four corners of your bed, but unless you are very tall, you'll have to step inside the bed to do lots of the planting, which kind or defeats the idea of a raised bed - makes it easier to reach things, and you don't have to tread on the soil, compacting it, which won't do the soil structure any favours. Not trying to tell you how to suck eggs, I'm just really curious as to why such a big raised bed?
