Unfortunately, I don't have access to a digital camera. Also, I'd better explain the situation a bit more clearly - I described the job as wiring an extension, whereas it's really wiring the whole house. In other words, I'm wiring one part of the house up at the moment. It's an old house with no electrics at the present. So I'm starting from scratch.
What we have at the moment is the supply coming in through the meter (with the 40A breaker) to a consumer unit which has a 35A main fuse and a 30mA RCD protecting a temporary outlet in the unit. The outlet also has an MCB. That's it. I've since spoken to a friend who has fitted electric showers in his house over here and he says that 40A is about the highest domestic supply you'll get out here. So it seems to me that it's a case of making sure that we don't put the kettle on whilst the shower's in use!
Although they go about things slightly differently over here, I'm using the Which Book of Wiring & Lighting as a reference (which brought the situation to my attention to start with) and, presumably, as long as all the relevant circuits have the correct protection, then the worst that will happen will be the occasional tripping of the RCD.
Thanks for replying so quickly.