Jill Goes Pony Trekking

This is the third book in the Jill series by Jemma Spark.
Jill Crewe is planning to go pony trekking with the Merrivales, reliving some of the idyllic moments of her youth. But she has this happy knack of falling into adventure and along the way she gets mixed up with an extremely handsome but mysterious young man. Then there are people after them and they need to disguise themselves before they dash to Dartmoor.