I'm aware that the IMC rating is only valid in the UK but at risk of sounding dumb (OK no risk at all...) what defines the UK? Does it include the Channel Islands? If it includes NI then surely it must cover Eire as well?
It covers N Ireland, and I believe dependent territories (Falklands?). The CI have their own ANO and an equivalent rating; I don't know if the IMCR applies there but it is moot since most of the relevant airspace is Class A.
Note that only the IFR privileges of the IMCR are limited to the UK. The fact that with an IMCR you can fly VFR without sight of the surface, and VFR down to 1500m (instead of 3000m) is not restricted geographically, and enables legal VMC on top VFR flight worldwide. I have this in writing from the CAA.
What happens if you go to France and have crap weather in France for the return flight? Are you stuck there?
Yes. You have to be legally VFR in French airspace, but from the airspace boundary (mid channel) you can be IFR.
If you set off for France in IMC and get half way across the Channel because the forecast was good for Dover/Calais but it stays pants do you have to turn back because you are now 'illegal'?
Yes.
With a full IR you would request (and get) an IFR clearance and proceed IFR and land with an ILS, etc.
While I'm thinking about it, does the night rating have the same rules as VFR ie in sight of the ground at all times except that somebody has turned the dark on?
Yes. A bit silly, isn't it? The NR (NQ) is an anomaly because if the night is 100% dark then you are doing 100% instrument flight, in terms of both aircraft control and navigation which now must be radio nav (GPS, etc). Yet you can do this on a plain PPL, with a few hours pottering about over some well lit terrain (Kennedy Jr. found out the hard way).
The IMCR is hugely valuable in the UK, for low level flight below Class A and landing with instrument approaches. In a very different way it facilitates long distance (VMC on top) VFR touring across Europe, but in the latter case you need to be sure of the weather at the far end, otherwise you will look a right pra*t turning up overhead Biarritz at FL060 when the wx down there is OVC006
I did a lot of VFR touring before I got the IR, and VMC on top really made it possible. Flying VFR below the cloud (the traditional UK VFR, due to low Class A bases) doesn't really work for long distances, due to a convergence between rising terrain and the cloudbase
The other thing is that while flying "VFR" in IMC is illegal, it is undetectable when enroute, and this is an added bonus which many pilots use to their advantage
But you need to be well sorted on the equipment/capability front.