We are in the process of building a secure network reference architecture for member firms as guidance. Are there any good examples I can use?

There's some material on this on NCSC's website.  One page of which is: Architecture and configuration - NCSC.GOV.UK and there is more if you search the site.    I've seen various examples at the different public authorities where I've worked over the years but they are rarely distributed widely.

I would focus on NIST and ISO/IEC 27001.

The NIST Framework - The National Institute of Standards and Technology (NIST) provides a framework for improving critical infrastructure cybersecurity. The NIST framework is notable for its focus on risk management and its adaptability, allowing it to be applied in a variety of contexts.

ISO/IEC 27001 - ISO/IEC 27001 is an international standard for information security. It provides a set of standardized requirements for an Information Security Management System (ISMS). The standard adopts a process-based approach for establishing, implementing, operating, monitoring, maintaining, and improving your ISMS.