If the garage you use didn't, as the very first thing, check the fluid level and visible condition of the oil then i would be sure never to darken their doorstep again.

Assuming its not fluid level or condition then another gearbox would appear the best solution, maybe one from a known breaker.

Hopefully someone else has an opinion or knowledge of the £475 exchange jobbie, it sounds too good to be true.

One assumes that this vehicle has a means of checking the gearbox oil level?

First thing to have done is check the level and top up (preferably refresh) if necessary.

Does this car have an oil to water gearbox cooler? Check gearbox oil for signs of water content.

£475 sounds cheap, I'd assume this is a straight 2nd hand 'box probably with unknown pedigree.
For a 2nd hand box with good pedigree, changed by a suitably knowledgable specialist or Ford independant, I'd have thought about £750 to be a fair starting price.