Where are you getting your turf from ? if I were you I would lightly rake the topsoil so your about level and using the heels of your feet walk over everything and compact, never use a plate compactor because turf wont grow right with too much compaction, you need to leave the turf to settle on its own to get the best results.
Depending on how much you want to spend depends on the results you will achieve like most things. I am a landscaper and I only ever use rolawn and occasionaly farnabies, depending on where in yorkshire your from you may of heard of it.
Rolawn and farnabies both sell a topsoil with blended loam in and its perfect for laying turf on.
Depending on the quality of levels you want to achieve depends on what you do next. With patience decent results can be given by finely laying topsoil with a rake or soft brush and laying from there but I always lay straight pieces of timber or steel bar to the finished height I need to be and then using a straight edge scrape over these creating a perfectly level bed, this is otherwise known as screeding.
When laying your turf always remember to lay in brickwork fashion and not lay cut next to cut and once laid if your standing on the turf lay boards over it to stand on as it will be very soft especially in this type of season.
One last tip, try to lay when we have a sustained dry period or wet period without frosts till the turf has time to root.