In Lancashire, we saw a Prunus avium tree covered in a clingfilm-like covering.

This is classic caterpillar infestation. Many organic or synthetic insecticides exist that would eradicate this problem. Any good garden centre would be able to advise. I prefer to use a pyrethroid-based insecticide against caterpillars. There is also a bio-control agent (Bacillus thuringiensis) that would prove effective. Again, I am sure this product is available in good garden centres.