Does anyone know of a way to get areas of a car which appear to be over cut with polish to come up reasonable again. At the moment the bonnet of my car has this from what i can see. I just want to try and get a consistant finish.

If it's a clearcoat finish and it hasn't peeled off or you can still see metal flakes, then you have a good chance it's saveable

best thing I can suggest - bring it to me - I'll have a good look and give you some rescue plans to try.
may need a machine to save it but will not know until I see it

Does the process vary for older cars ? How would detailing a 1970's model car go ?

With 70's cars that are acrylic, you can use the same compounds and polishes and use a rotary buffer at higher speeds than with a clearcoat finish but it is easier to burn the top coat off which then leaves the car with a dull finish

Also the pads get clogged up with the oxidised paint so whatever colour your car is, the pads will end up looking like that and they will need to be rinsed clean by hand