I would seriously look at getting a quote from a different service provider before biting the bullet for a replacement truck.

£6k for a radiator replacement? Someone's seriously chancing their arm here and is downright theft if that's all that needs to be done.

A brand new radiator is going to maybe cost £600-£700.
Maybe 4-5 hours to fit at an average independant company labour rate of £60 an hour plus a bit for travel and antifreeze/consumables and I'd expect a bill around £1k give or take.
£6k implies about 80hrs labour!

Is the rest of the truck in OK condition?

Please provide some more details on what was quoted as myself and most likely all of us on here hate to see customers get ripped off!