It's possible that:
* The name is misspelled: Double-check the spelling of the name.
* She uses a different professional name: Perhaps she goes by a stage name or uses a different variation of her name.
* She's not a well-known figure: There are many talented actors and actresses who work in smaller projects or haven't achieved widespread fame.
If you can provide any more information about Sheila Brownrigg, like her potential age, genre of projects she might have been in, or any other identifying details, it might be easier to find information about her.