Sorry, but I wouldn't waste your time. Leylandii will shade and have lots of surface roots which will suck the moisture and nutrients from your rhubarb. Rhubarb is a greedy feeder, needing good light, plenty of moisture and lots of feed, including rich compost and manure. How about a nice statue or shade tolerant plants in containers?

Euphorbia robbiae is about the only thing I can think of that will survive in the ground there.There are several varieties, some run and some self seed and they all have long lasting heads of limey yellow flowers in spring. If you plant these don't get any of the white sap on you skin and especially in your eyes. its a very useful plant for dull dry corners.