This paper discusses the quantification of the current risk of mid-air collision to commercial air transport aircraft receiving a Radar Advisory Service in UK Class F/G airspace. The aim is to produce the best estimate, given published data, and then to indicate what extra studies and data would be needed to improve the estimate. A study of typical flight routeings by certain aircraft types, e.g. Sikorsky S76, Saab SF340, Short SD3-60 and Aeritalia ATR42-300, could provide useful additional information. The fitment and use of ACAS could be crucial in delivering the required protection against collision.