Who decided to drive on a motorway with a gearbox leaking loads of oil?

The garage or you?

Answer that and the repsonsibility lies with your answer.

Without more detail, i'm not really sure there is anything you can do. If all you asked the garage to do was find out how much oil was leaking out, that is probably all they did. A couple of points though, oil spreads very far, especially in damp conditions, so the fact you were seeing what you felt were large patches, does not mean that it was a lot of oil. Over a long period of time, the gearbox oil eventually empties, and gearbox then goes pop. But the point is, if the garage simply checked to see how much oil was leaking, saw it wasn't actually much, they probably didn't do anything else. Maybe they should have checked the oil level in the gearbox, but that is where your words to the garage is key. Even if you told them you did want a thorough investigation into it, it would probably still boil down to your word against theirs.

How old is the car?, you haven't told us anything about it other than its mileage.

no chance, the gearbox oil was all over the underside of the car and the back of the car when it was recovered.

I can see both sides here. When oil leaks it tends to go everywhere and can be difficult to identify source without cleaning it all off and monitoring again.

It maybe the case they didn't identify the source of the leak correctly or maybe you had two leaks. I suspect it was a driveshaft oil seal leaking which went all over the gearbox and sump.

The garage may have mis identified it although if you're honest with yourself if you'd tried to fix it when it first started then you'd have seen it still leaking well before it ran out of oil.

Assuming it is a regular manual car I'd try and find a used box. The clutch and DMF may be OK - they're quoting a new one as it is sensible to replace with all the gearbox out. Depends on how long you plan to keep the car.

Although a tad incompetent all you asked the garage to do was rectify the leak which they did. They weren’t asked to check oil levels

While you're right, that shows a pathetic level of mechanical intuition - even I would think to myself "If some has got out, some needs to go back in". And I do nothing except basic service items myself.

2011, FSH. I have only had it a year

I never 'assumed' anything. The garage said I had an oil leak and that it was the sump plug. Is that not engine oil?? There was no oil light on at any stage.

The gearbox oil is completely different to engine oil, as I now understand.