Unless the clay is so bad you can make pots with it, a better option would be to dig it over, incorporating as much humus rich material as you can afford at the same time, and that's things like composted animal manure, soil conditioning compost from the garden centre, home made garden compost, leafmould, spent mushroom compost, anything like that. Also, if your chippings are pea shingle and they've been down more than 2 years, dig those in too. Clay has higher nutrient levels than sandy light soils and will grow a wide range of plants successfully, not to mention that, unless you pay absolutely top dollar to a reputable and reliable supplier, any topsoil you buy in will likely be clay motorway spoil.

clay is really bad all lawans in the area are laid onto layer of membrane with top soil on top neighbour had theres laid right last year and had to get excavator in to remove the clay it was solid. So think digging it in will be a no go any idea on how deep i should go and i will have to try and find good top soil
tahnks
alex

At least one spit, which is a full size spade's depth.

Clay soil has some good qualities because of its density it retains moisture well, it also rich in nutrients I have heavy clay and have worked on it over the years digging 2 spade deep and adding plenty of compost, leaf mould, grit etc and Vitax clay breaker. Also I tend to go for plants that do well in clay soils. We are very proud of our lawns on clay around here but it is the work you do before turfing/sowing that makes the difference together with on going maintenance.