Sionainn
I can't say for sure, but I do know the average print run for a new book from a larger publisher like Penguin is roughly around 6000 here in Australia. Whether or not they did a second print run with that cover or not, I can't say either.
Since it was published back in the 80s though it could be less, and being children's/YA would effect the size of the run as well. The Literature Board also supplied funds for the title to be published, so I imagine at least 2000 cause of that, but again, not really sure how things were in the 80s.
blackbird
My copy is a second printing from '88, so there - theoretically - should be a few floating around?