Just as an FYI, if the cables aren't long enough due to the length of a dual-RCD board, you can always opt for a (albeit more expensive) small main-switch board with RCBO's. I've found in the past that there's rarely a need to extend anything then, plus you're providing the best installation available.
If you do need to extend, you could try the new LineProducts in-line push connectors (see sponsor ads).