It's going to be costly,, bent valves maybe damaged pistons
It's a cylinder head off job then a trip to an engine reconditioner to change any bent valves and pressure test, new gaskets and water pump amd timing belt.
Plus 8 hours labour @ £40 if your lucky
I wold say £800 plus vat
All for the cost of a rubber belt and 2 hours to fit its called preventative maintainance amd changed around every 60,000 miles