is it safe to use rhubarb leaves on a compost heap

Yes...why shouldn't it be? Rhubarb leaves are poisonous of course, but that doesn't mean you can't compost them....

Or does it? I bet someone will give me a really comprehensive reason now for not composting them LOL

Nope they are fine on the compostheap, great too for laying around plants/veg to keep weeds down and the soil moist.

That's a good idea Tillia.......