Can someone explain what this is

This pretty little flower is growing low down on a fairly new David Austin rose. But it isn't a rose. Anybody explain what has happened.

Quiet day, nice chat with my sister, a good session of deadheading roses, which is why I spotted this little intruder. Then the Federer match! Great result!

Now I'm breaking all my usual rules....I am sitting with Jack waiting for the England/Ukraine game to start.
I might watch all of it or wander off if I get bored.