> My boss then says "bad news, its been renewed already"
> Equally my customer ... said they had not signed on the line yet
Taken together, the above two mean that your boss has lied/misled/not been entirely truthful with you.
If there is evidence that this has been done specifically to disadvantage you then you could have a case.
If there is evidence that this has been done specifically to advantage the company then you could have a case.
If it's general company practice to renew as early as possible, and your cover has gained at your expense but not to the advantage of the company, then it's still possible that you have a case, but less likely. You would need to check your contract to see what it said about maternity pay, especially whether it was based on basic pay or total compensation.
> I said I would leave it to him to decide what to do, so that no-one can ever accuse me of working against the business.
They already could, as the conversation you have had with your company's customer [not strictly YOUR customer] could prejudice that customer against the company. That said, if you are that close to the customer, I would encourage them to wait for your return so that it is clear to your company that much of its relationship with their customer is based on their customer's relationship with you personally.