Mimulus guttatus is my thought.

Thanks for that it still surprises me the variety of plants that come up in my garden without me planting them.

Given that you are in Nottinghamshire, you should consider reporting the 'find' to the Non-native Species Secretariat.
It has not yet colonised the East Midlands. It hasn't been declared an invasive species yet but it is beginning to (seriously) establish itself.