england-and-wales
I entered into a contract to have my sewer line replaced, and in the
acceptance of this contract, I stated that my homeowner's insurance
was covering the cost. I told them in the acceptance email that I had
paperwork for the claim for them to fill out.
That isn't how acceptance works. Acceptance means that you accept the terms that were offered to you without modification. If you incorporate additional terms (e.g. requiring them to complete insurance paperwork) then that is a counter-offer. A counter-offer has the effect of extinguishing the original offer (making it no longer capable of acceptance), and it is then up to the other party to accept your new offer. With that said, acceptance can be by conduct. If the contractor turned up to the job after receiving your counter-offer, that could be considered acceptance.
I don't know how you worded your email. In the alternative, it could be that you gave a clear and unconditional acceptance of their original terms and with a separate follow-up request to complete paperwork which wasn't linked to the acceptance. In that scenario the request isn't part of the contract.
I feel this is a breach of contract. Do I have a leg to stand on?
This depends on the exact wording of your email. If your email constitutes a counter-offer which states that they needed to submit documentation to the insurer within 1 month then there could be a breach of contract. If you made a counter-offer which didn't specify a time limit then the situation is more difficult. You may be able to apply Section 52 of the Consumer Rights Act 2015:
(1) This section applies to a contract to supply a service, if—
(a) the contract does not expressly fix the time for the service to be
performed, and does not say how it is to be fixed, and
(b) information that is to be treated under section 50 as included in
the contract does not fix the time either.
(2) In that case the contract is to be treated as including a term
that the trader must perform the service within a reasonable time.
(3) What is a reasonable time is a question of fact.
The fact that you appeared to allow them to carry out the repairs after 1 month despite not having submitted the insurance paperwork might suggest that the delay was treated by the parties as being reasonable. It might also be reasonable for the contractor to wait until the works were underway or complete in order to have the necessary information to be able to complete the paperwork accurately. As noted in sub-section (3) above, it will be for the Court to decide what is or is not reasonable based on the circumstances of your case.