Now this is very technical, making a dibber......

Is it for leeks, parsnips or what? Whatever, dib a 6"x3" hole and fill with water; do another hole twice the width and do the same again. This will give you a better idea as to your drainage. You are right that, if your soil doesn't drain well, you will be making a sump for water to drain into via adding compost to a firmed-in soil. I haven't heard of adding granite dust, but tons of sharp sand should have improved your general drainage significantly - how deep did you dig it in? Remember that the ground is soaking wet (I presume yours is?), if you live on low-lying land you may have a high-ish water table. Tou may have a hard clay "pan" beneath the top soil where it has been uncultivated and this needs to be broken up by trench digging and breaking up the bottom layer and incorporating your grit, then backfill with the top soil adding more grit and manure/compost as you go, but don't get the sub-soil onto the top.

Oh Crikey!!  What you have is inert soil from the garage base, plus any original top soil there (quality and fertility unknown and has never been previously cultivated) plus "granite dust" and sharp sand. Is the water table a general one for your area or just what you found in the bed? It sounds like you have both a huge draining and soil quality problem and digging to one spade depth will not solve it. From your post, I gather that the bed was 18" high and now raised by digging etc to 2'6" - this will sink a lot over the next few months. This might now be better posted in the "Grow Your Own" or "General Gardening" Forums but all I can suggest is to either - make large dibbing holes about 4" diameter, fill with good compost, perlite and sow your seeds, or make a "lasagne bed" on top or  - the quickest but hardest way - is to remove as much soil as you can, order some good quality gritty top soil, get a big fork, make a trench and push the tines into the bottom whilst adding sharp sand and grit, then add the new top soil with as much compost as you can.