Although too early to be certain of the caterpillars identity, being on Hawthorn suggests they are likely to be the larvae of the Orchard Moth Yponomeuta padella – see more on caterpillar webs
Should this be the case then over the next few weeks these bushes and any vegetation nearby may become covered in webs.
Any plant or shrub in their path will be covered in a fine web.
Even fully grown Lackey caterpillars such as the one shown right may become part of the web if stationary for long enough.
Other moth caterpillars can be seen in the gallery of British moth caterpillars