I take all these responses under advisement .. But ..

1 At the end of the day the use of the phrase 'modify an existing reservation' must contravene the UK Trades Description Act . because you cannot !! All that apparently happens when you make a material change to a reservation is that Hertz tear up the old one and generate a new one .. you pay the new price .. if it's more than the old one .. tough

2 Not allowing me to rent a car for 7 days for the same price as an existing 9 day quote is just .. bl**dy mindedness (I was told that if I didnt show on the original start date the rental would be cancellled)

3 This bl**dy mindedness could encourage me to reciprocate ... I might just go to Avis but forget to cancel the Hertz reservation. Maybe even a denial of cars attack .. book 10 cars and ....