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Yes, if you can show you contributed then you can establish ownership - if there is no partnership agreement in place, the law says that you are still a default partner (even if there is no written agreement) so you are a 50% owner in that scenario. The law is under the Partnership Act 1890 - so if you and your ex work in a business designed to make a profit then you are deemed to be partners regardless.
I hope this helps.
If you worked for the business with someone and it was to make profit, by default you are a 50% shareholder (this is why it is vital to have a partnership agreement drawn up to specify share amounts)