does any one know if conifers that have been shredded up can be used to dig in clay soil at all
I wouldn't have thought so - conifer takes years to break down.
Unless you are on a clay over limstone, the last thing you want to do is dig in something which is going to acidify the soil. Generally speaking one adds lime to clay soil to help its structure.
Great stuff as a mulch, but put it on your compost heap, it will be great next year, just the same as recycled xmas trees.
I'm with Ace... Mulch and compost... store in old compost sacks for a year or so!
;D