I would prepare your heavy clay soil well in advance of turfing (at least 6 months, ideally), by incorporating composted bark and grit. I would also add a thick layer of top-soil so that the turf gets the best possible start.

Many thanks. As always success depends on thorough preparation and patience I suppose!

Use boards to walk on - old floorboards or scaffold boards are perfect. I've used old shelving in the past, but if its melamine, tends to be slippy, so care must be taken so you don't break your neck. Using boards on heavy soil, specially once you've prepared it but still need to walk on it, spreads the weight and reduces compaction considerably. Once you've done the initial dig, not walking on the parts you've dug, so working backwards, when you need to go on it again, that's when you use boards. They don't have to be suspended, you can just lay them on the ground where you need to stand, raking up the surface afterwards. Essential when you want to lay the turf too, for kneeling on...