Robbie did a good job of tracking down the information which is 
available on line as text. Alan Pell died several years ago, but he 
retired from the organ business before that, so trying to contact 
him will be unproductive.

I found a Worthpoint listing for a Harmonette which sold sometime in 
the past. That listing has a photo which includes a manual as part 
of the listing, so a instructions were once provided.

The cover says Harmonette Instruction Manual, and indicates that it 
was produced by Melvyn Wright, who was a co-developer of this little 
gem. Alan Pell was neither a prolific author nor did he have Melvyn's 
software skills, so this was probably the prime textbook.

Melvyn might be able to provide a copy, but is unlikely to see a post 
on MMD. His massive website is http://www.melright.com/busker/index.htm 

It might be possible to obtain information from Rob Barker. He helped 
build a lot of these organs: https://robbarkerorgans.co.uk/ 

Both Melvyn and Rob operate one-man businesses via the Internet, so
so don't expect an instant response from them.

Wallace Venable