Helen Solomon wrote books for the younger reader. I haven’t been able to find anything she wrote since Elsie in 1989. At least two of her books have some pony content: the intriguingly named A Pony from the Pit has proved impossible to find, but I would imagine it is more pony-orientated than Stranded at Staffna.
Finding the books
Stranded at Stafna is not impossible, but pricing is variable. The book had no American printing I can find. A Pony from the Pit is very difficult to find.
Links and sources
The British Library