First, check very carefully what is written on the invoice for the gearbox replacement done by your local garage. The brake fluid can only be leaking from the clutch 'slave' cylinder (this is the part which actually operates the clutch mechanism when you press the clutch pedal).

They should have replaced the slave cylinder too, and this should be listed on the invoice. If it is listed, then go back to the garage and tell them to inspect the car as the replacement slave cylinder seems to be leaking.

Also, you may need a clutch plate replacement if the slave cylinder is leaking (because the leaking fluid contaminates the clutch plate). I am surprised that a reconditioned gearbox was fitted. I can't see how a leaking clutch slave cylinder ruins the gearbox.

As far as I know brake fluid cannot leak fom the gearbox.

However I believe that the Meriva has a clutch slave cylinder which may have been leaking hydraulic fluid and would have required gearbox removal for replacement.

If this is the case it is likely that the replacement slave cylinder has failed again or was incorrectly installed on the original repair.

However if a gearbox gasket had failed and was leaking transmission fluid, this may require a gearbox replacement. Rebuilding an original gearbox to replace the gaskets is a fairly specalist task which most local garages would be unable to do.

So firstly - be clear what the original fault was - slave cylinder or gearbox.

If the current fault is one which should have been fixed originally you may have a claim that the garage that the work was deficient. However I suspect after nine months you are on the margins of what they are statutorily liable for so start the dialogue constructively - don't go in hobnail boots flying!