Can a beneficiary demand to see receipts and vouchers backing up an estate trustee's accounts?

Yes. Beneficiaries are generally entitled to see the records backing up an estate trustee's accounts, not just a summary of totals — receipts and vouchers are exactly what a court expects an accounting to be supported by if it's ever formally passed, so a trustee refusing to share them with a beneficiary who's asking reasonably is on shaky ground.

If informal requests for supporting records go nowhere, a beneficiary's recourse is to push toward a formal passing of accounts, where the trustee has to file a properly supported accounting with the court and provide it to everyone with a financial interest, including the underlying documentation for anything a beneficiary wants to question. A trustee can't simply assert that expenses were proper without being able to back that up if challenged.

This is also a good practical reason for trustees to keep organized records from day one rather than scrambling to reconstruct them later — a trustee who genuinely can't produce receipts for significant expenses when asked is likely to face real difficulty getting those items approved, and possibly personal cost consequences, once the matter is properly before the court.