John Herbert Cowin (Bert) was a former employee of Willis, who set up his own business in the mid 1950s.
I have copies of correspondence from the firm to a customer, in 1959, which is headed with two addresses, both in Liverpool. One in Mayville Road, which would appear to be an office or home address, the other, noted as "Works," is in Lucknow Street. However these may not be the only addresses associated with the firm.
It appears that J. H. Cowin had quite a few employees, but he died while in his mid 50s, in or around 1965. The firm is no longer in existence, but his nephew, Keith Edwards, is an organ builder based in Cheshire.
Thanks to Quentin, and Keith Edwards, for their help.