I assume the garage tightened all the hose clips and connection?

but:-

The radiator is 16 years old! Its well past its use by date. The radiators rot and fracture at the inlet and outlet pipes. Removing the hoses often creates the fractures ... its rusted, fatigued, and been heated/cooled in 16years of driving. Get the radiator recored for about £100 and it will be as good as new. Check telephone pages for a local motor-rad depot.
Replacement top hose is often difficult to obtain. Match it against any other common hoses. Usually the 'bit' needed can be cut from hose available at any diy motor shop. Much cheaper than diahatsu parts.
You will have not luck getting your garage to pay for a new radiator. They changed the timing belt as requested. Any other dodgy bits are not the garage's problem!

Your theory could be a factor but it is more likely that the Mechanic damaged the rad when changing the timing belt. The Radiator and associated piping and shroud have to be removed to gain access. It is a bit fiddly getting the combination of Rad. shroud and pipes past the fan which is then removed . If the vehicle rad is old then they become very fragile and damage can be done quite easily unless the mechanic is carefull and takes his/her time to remove.

As you say it is possible that repairing one leak may produce or show up other weak areas but highly unlikely to the extent youu describe.

My advice is to take the car back to the Garage and get them or him/her to rectify the problem which was caused by them. I assume they paid for the part they broke!

A new Rad for a FEROZA ( Sportrak) can be purchased from a Company on Ebay for just over the £100. I have one and apart from an additional outlet on the bottom tank they are superb. The extra out let can be sealed with a bit of heater rubber hose, 15mm copper tube and end cap plus a jubille clip.

OLDMINIMAN