The TRRL DCPT is one test used in road works that "correlate" to the in situ CBR.

You can also perform a CBR test in the field directly.  This is usually equipment mounted on a truck bumper.

Thanks, I am actually thinking about what I could do in the field other than the CBR as I'm expecting the ground conditions to consist of a mixture of gravel and cobbles (plus 20mm), so it may not be possible to carry out an in-situ CBR. By substituting a DCP (which I think is generaly just used for existing roads) I was thinking that if cobbles are hit during the test then this would render the test unsuitable too. Any thoughts?

Trouble when you are in granular with cobbles, etc. hardly any test will be reliable.  What do you need the CBR for?  Typically, the CBR >30 is standard if for granular subbase.  Fang (Foundation Engineering Handbook - the old Winterkorn and Fang) has correlation chart in it showing CBR vs classification, CPT/SPT, Mod. of Subgrade Reaction, etc.  Check it out.  Also Baker Highway Engineering Handbook has another correlation chart (similar) but using more highway oriented tests like the R value, etc.