Thanks for the feedback above, all good advice. We'll book direct with Avis and save ourselves a fair amount of cash which we can waste on enjoying ourselves and spending the kids inheritance lol.
Our rental is for 30 days so we are just over the maximum 28 allowed.
We do have Avis Preferred and also excess insurance through icarhireinsurance. It covers windscreen, tyres and the underneath etc which is essential for driving in Namibia. We were there earlier this year but hired a vehicle which was fully equipped with camping gear etc. Luckily we didn't have any issues even after driving some 5000km in five weeks.
Next year we are going down the "staying in lodges" route so don't need the same kind of vehicle we hired this year, hence the attempt to book via BA.
Great advice re the petrol stations, hubby always gets out of the car. We've lived in Africa in the past and also try to go back at least once a year, especially since we discovered the delights of 2-4-1 companion vouchers with great advice from the folk on FT.
Thanks everyone. Happy travels.