I see that under the new version of FSMA, complainants will not have to accept an Ombudsman's decision in writing.

Nor will failure to respond within the specified time limit amount to a rejection. A new subsection will say:

"(6A) But the complainant is not to be treated as having rejected the
determination by virtue of subsection (6) if
(a) the complainant notifies the ombudsman after the specified
date of the complainant's acceptance of the determination,
(b) the complainant has not previously notified the ombudsman
of the complainant's rejection of the determination, and
(c) the ombudsman is satisfied that such conditions as may be
prescribed by rules made by the scheme operator for the
purposes of this section are satisfied."

In other words the Ombudsman can decide the uphold should apply anyway despite the complainant not responding in time.