UK Auto Repair Questions?
Hello, I hope the following is of assistance.
It's never a good idea to add any kind of "radweld" type product to a cooling system, this is really used as a temporary measure by breakdown services to assist at the side of the road.
If used the product by it's nature blocks holes up, this is how it seals the leak, unfortunately it also tends to seal holes that are supposed to be there such as the water ways through the radiator and heater core.
In the long term this can result in problems with coolant circulation and can lead to the engine over heating.
Whichever component was leaking should be replaced and the cooling system flushed to remove the sealing product, this is the correct repair procedure, the garage will have used the sealant product to save money and avoid the work and cost required to replace the leaking component.
I hope this helps but if I can assist further let me know and I'll reply as soon as I'm able to.