Hi Alan
We had the same dilemma with our boat when we installed a 1000 watt inverter. I have wired this to a ring main with 4 double sockets around the boat, and then wanted to add shorepower. Having spoken to Brian Ward's in Brundell, there are basically two alternatives to the issue of switching. They sell a manual change-over switch which selects between the inverter and the shorepower for around £35. The more expensive option (over £100 I think) is an automatic changeover switch, which detects which source is providing power and automatically switches to that.
I haven't yet got round to installing the shorepower, just bought all the bits, but I also am not sure about where to put the RCD and consumer unit in the circuit. I am assuming it needs to go after the changeover switch so that the ring main is protected whether on inverter or shore supply, but as the inveter can only supply about 4 amps I'm not sure about this.
With regard to running the engine when the inverter is on, it really depends on what you are powering. Our LCD TV runs from the inverter, but is only 60watts and therefore uses not a lot of battery. However a microwave will knacker a battery in 10 minutes, and hence we split our battery bank so the inverter ran from a dedicated battery. Even if that one goes flat, there are still 2 domestic batteries for the lights & fridge etc, and the engine battery is separate as well.
If you are ever in the area of Ludham, and wany to see what I did with our boat, let me know and you are more than welcome aboard!