Garage consumer unit

Hello You don't necessarily need a rcd in the fuseboard in the garage as its protected by the RCD in your main board judging by what you have described. Replacing a fuseboard is notifiable work under Part P of the building regs, so the electrician doing the work will spec it for you and give you certification at the end of the job, this certificate would be needed if you ever sold the property etc. Hope the info helped. Ricky Mockford Lancing Electrical Services

A 40 amp 30 milli amp RCD the 40 amp bit is the maximum load that RCD can take. If your maximum load hasn’t changed another RCD of the same spec will do of course you want a double pole RCD. You can have another RCD down line as long as it’s a latching RCD otherwise if a fault were to occur or a power failure the RCDs will trip. I advise for a consumer unit change call a professional qualified electrician and get few quotes.