Ok guys following your advice with sourcing used or new old stock and a chat with a retired Electrician, I have decided to have the CU converted to a double RCD unit. This is bearing in mind that the initial EICR recommended having the lighting put on an RCD circuit too.
We have found this RCCB on eBay, see photo, and have a photo of the CU with cover off. However, I have noticed that they look different despite the fact that the serial/product numbers on the RCCB's are the exact same. I cannot find another one like this and also there is a strange, non-switched, unit to the left of the RCCB with a L sticker on it. Is this a part of the RCCB or some other kind of device?
Thanks...