In reply to mark237: Yep, harvest mites - either the same or very similar to chiggers in the US. Only way to reduce chance of getting them is if you're in an area where they're common (limestone crags in late summer/autumn), have a hot soapy shower as soon as you get home. Don't leave clothes lying round on the ground and don't sit down.
The red lumps aren't bites, they're where the larvae have attached to your skin, injected their saliva which liquifies your tissues and they then insert a small tube to feed. Hot soapy shower dislodges them.
Limestone areas seem to be worse, where there is long grass or dead leaves on the ground - they don't like bright sunlight so hide under the shade of these.