Many thanks for all your postings of support. Unfortunately he'd stored the serial number on his hard drive but not backed it up. It failed recently and he hadn't put it onto the new one.
He said he was going to E-mail the picture to Hobgoblin in Nottingham. I'll ask him to read your other suggestions too.
Having had my own concertina for several years I hadn't realised the prices they fetch nowadays. I shall have to photograph it as suggested and maybe get it valued and if necessary insured separately from the other house contents.