Can you cultivate the ground with a rotovator maybe, and remove any large stones as you go with a mattock or pick? Then maybe you could get in some topsoil and incorporate that into the existing clay soil or get in some sand, rotavate that into the soil and then topsoil over that. Excavating the top layer will most likely require a skip and like you say £££ and depending on the size of the area create a LOT of extra waste and work
Also if you haven't used a digger before it can be a bit if a struggle to pick it up in the short space of time you would be hiring it for...and if you want a man with the digger, thats going to double if not triple the cost.

I guess it depends on the size of area you are working with?

Depending on the size of the area, the cheapest thing to do is elbow grease - you'll need a rake, and good sized garden fork and possibly a spade. Rake off anything that's loose in terms of chippings, etc., then dig it over, removing bits of concrete, stone and general debris as you go. Once you've done that, level it off to determine whether you'll need topsoil or not - if its not much lower than your patio, you may not need any. Depending on what was there before you started building the patio, you may need to put in a few bags of soil conditioning compost from the garden centre to get the soil fertility up, and re level again. Clay soil isn't a problem in itself as far as growing grass is concerned, but the addition of the compost will help with that anyway.

When I layed my patio a few weeks ago I dropped the level where the slabs are going by the depth of the slabs plus 3 inches. I then leveled the area with sand (to a depth of 3 inches) The slabs then go on one by one. My base soil is also clay - so strong enough for a patio to be put on directly without cement. I would add a sandy-cement mix between slabs - I did not and have grass seedlings growing all the time.

thanks alot for all your advise, i will probably rake and pick up alot of stones by hand then rotovate the ground raking and picking stones as i go then take bamboo's advise and use a conditioning compost and top soil ready for turf. thanks alot

I've got one other thing to say if you're using a rotavator - dig out any pernicious weeds such as docks, bindweed, nettles, dandelions, etc., by the root, getting all the roots out before you use the machine - the rotavator will chop the roots into tiny little pieces and each one will grow again if you don't.