Given that Cranfield and Oxford are over 30nm apart I'm not sure how a joint radar service could function given that the best place for an airfield radar is on the airfield. The cost of remoting the information to one or both (if the head was centrally sited) could be prohibitive. Take a look at the elevation profile between the two airfields too. The highest point is about 12 miles southwest of Cranfield which would provide reasonable cover, but probably not below a couple of hundred feet at either airfield.