
Not to worry, I just worked it out (seems obvious now, apologies).

#Function for generating parameters
genparms<-function() {
list(
x = c(runif(1,2,3),runif(1,2,3))
,y = runif(1,2,3)
)
}

#Create N parameter sets
N<- 10
set<-rep(list("NA"),N)
for (i in 1:N) {
  set[[i]]<-genparms()
  }

> From: james_980@hotmail.com
> To: r-help@r-project.org
> Date: Wed, 28 Nov 2012 06:28:03 +0000
> Subject: [R] generating sets of parameters when some parameters are vectors
> 
> 
> 
> I'm trying to generate N lists of randomly selected parameter values. Some of the parameters are vectors while other are single values. I've tried to use the approach below but it doesn't work for parameter x, which should be a vector. Thanks in advance! 
> James
> 
> 
> 	  #Define names of parameters in the list,
> parms<- list(
>   x=c(0,0),
>   y=0)
> 
> #Create N sets of randomly selected parameter values
> N<- 10
> set<-rep(list(parms),N)
> for (i in 1:N) {
>   set[[i]]["x"]=c(runif(1,1,2), runif(1,2,3))   ###Problem here
>   set[[i]]["y"]=runif(1,5,10)
>   }
>  		 	   		   		 	   		  
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> 
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