The newspapers have reported that the McCanns have consulted Michael Caplan, QC and extradition law specialist. These reports also rightly point out he might not be of much help as any attempt to extradite them will almost certainly be under the European Arrest Warrant.
This is a device designed to simplify cross border justice in the EU and there are a limited number of criteria under which it can be contested in comparison with a traditional extradition request. Firstly the judge will have to decide if on a balance of probabilities the person in front of him is the person the warrant is for; in the specific instance of this happening to one or both McCanns I can't see this being an issue. Then we get to the big difference compared with most countries which we have extradition treaties with. There the judge must decide whether there is sufficient evidence to make a case to be answered (if they have yet to be convicted, in that situation other rules apply) this does not take place in an EAW case.
The only grounds to prevent extradition using an EAW are if the offence shouldn't be subject to a warrant on grounds of its severity, double jeopardy applies, persecution on the grounds of race or a number of the usual prejudices, the request is out of time or the person was too young, medical or physical conditions or the lack of suitable legal arrangements (including measures under the International Convention against the Taking of Hostages). Otherwise it is assumed that as the UK has declared this country to be competent to issue the warrant that their criminal justice system is of a high enough quality to not to endanger the human rights of the person the warrant has been served on.
What will be very interesting is what the more excitable sections of the press will say if the McCanns are subject to an EAW. The legislation that enforces them in UK law is the 2003 Extradition Act, the same act that brought in similar changes (again dropping the requirement to present a prima facie case against the person concerned) to the extradition situation with the USA; The fact that they haven't reciprocated caused very little in the way of widespread public debate until the case of the Natwest 3. At the time the EAW was proposed The rabidly anti-EU newsprint used it as an excuse to attack judicial systems based on the Napoleonic Code and the concept of examining magistrates on the grounds that they weren't British and therefore inferior, but again it didn't see widespread debate amongst the public. So which way will they jump?