Sign into webmail and create a Filter Rule similar to the following:
For the Regex field value copy and paste the following text [Ѐ-ӿ]
FYI: Provided unique flag colours are set with each rule it can provide a visual indicator of which rule ran against a message; there are actions settings like Add IMAP keyword but I have no idea how they would be used in webmail.
As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.
@Fiona_W, you may want to disable the filter rule as it is making some unexpected matches against emails not containing Cyrillic text; only seems to happen when recipient quotes sender's email in their reply.