Can you give us a little more info? Is Cranfield radar controlled and were you in controlled airspace? What sort? E? D/C/B? (I don't have a map here.)
What sort of service did you formally get from Cranfield? RIS, RAS, FIS? Did you get a squawk? Mode A or C? And I assume you were a VFR (SVFR?) flight?
Was your QNH/QFE setting correct, were you at the correct altitude/height for your clearance?
Were there any other relevant communications related to this airprox, before or after it happened? Specifically, if you say that you were both aware of each other, was this because ATC gave you traffic information or was it circumstantial, pieced together from the others' radio transmissions?
Did you get the callsign of the jet (don't have to reveal it here)? Might be really useful for the report. Did you write down the exact time - same reason.