If it's the case, as I believe, that there's no rot on the car at all apart from this bit, it seems to me like it needs to be done properly, or the whole car is "spoiled". The problem with that is it's going to cost a fortune. Quite the dilemma, especially as the value of my specific car isn't huge, unlike a 1968 Cooper S mentioned above!
Why not get a 'satisfactory' possibly patch repair done for a reasonable price if the value of your specific car isn't huge and continue to enjoy the car, it certainly won't "spoil" the whole car. Such a repair should be fine for many years and in the unlikely event that it needs doing again in a few years you can do the same minor repair again....or if the value of the car has risen to '1968 Cooper S levels' by that time you may feel it is worth replacing the complete panel at the much higher costs quoted.
Edited by mab01uk, 30 April 2023 - 10:28 PM.