Thanks Redhand, that's a lot of ifs, but relevant, so let's assume wet concrete (ridged by tamping, not smooth), Grabber 245 tyres on 20" with 2mm min tread and inflated to suit the task (whatever that is), two 100 kg bodies up front and even distribution of no more than 200 kg tied down in rear, no more than a gentle breeze up the slope, slow speed (no rush up and descent control down). Obviously, can't have the wrong sort of leaves on the surface!
Look forward to the possible definitive prognosis!