Well I'm not Derek, but I'll answer anyway - Derek may answer himself as well later, hope he does, I'll be interested to see what he says myself.

No, don't spread top dressing and then turf. For one thing, you don't know the ratio of sand/peat/loam within it, and if its high sand and your soil is already free draining, it won't make a good base for grass, plus the texture of top dressing is light, given its intended to be brushed over existing, aerated grass.

The soil should have been prepared by digging over, removing weed roots and large debris such as bricks and large stones, levelling, walking all over it on your heels to get out soft spots, then re raking and levelling, till you've got a level surface which is friable for the top inch or two. Just before you lay the turf, broadcast Growmore granules over the area till it looks like light snow, rake lightly, then start laying the turf.

As you've got so much top dressing, unless you want to reserve it for use later for its intended purpose, you could spread some over the area prior to digging it, so that it gets turned into the soil before you lay the lawn.

Hi, I was wrong to make the assumption that as you mentioned top dressing, you already had a lawn, but I agree with everything Bamboo has said, nothing to add really, good luck with it, Derek.

Remember to water that turf alternate days until it gets settled.