What to grow

Perhaps if instead of soil you added something more moist like home made compost or manure to retain moisture, then I see no reason why your choice of plants wouldn't be ok. Intriguing slabs, maybe someone's covered a driveway.

You could put flower seeds over it in spring or plant some alpine/rockery plants but they like well drained gritty soil. Miniture bulbs would work for spring colour such as miniture narcissi Tete e tete and little gem, dwarf tulips, crocus, mascari, anemone St bridgid and De caen Snow drops unless planted in the green unlikely to flower first year. well worth visit to garden centre as full of bulbs at the momment. You can also plant other bulbs for summer too. One other thought is you could gravel it and add flowering pots to compliment the evergreens.

Sedums (small varieties) or Sempervivums can survive in very shallow soil. They are used on green roofs. They should also spread around at the base of the conifers. Or put troughs there with bedding. Don't put extra soil around the trunks of the conifers as this would kill them.

The three of you have given me so many suggestions an ideas thank you kindly. Regarding the "slabs" probably not slabs - as a neighbour told me, more likely builder rubble, well I am not going to dig further just to find out, got enough aches and pains fom the work, but it's all worth it

I like the gravle idear and the pots'that would mean that the conifers would not rob your plants of water and what ever you feed them. also the teracoter would add to a nice colour contrast with the trees.