Hi June - welcome to the forum.
What you desribe is typical if a high pressure fuel pump failure on the Dci engines whereby metal particles form the pump contaminate the complete fuel system.
When this occurs the complete fuel system including the fuel tank must be stripped and cleaned. Renault dealers insist on renewing all parts rather than cleaning so you can understand the dealer prices of around £3500 plus.
A good independent with experience usually charges around £1500 or so. They replace the fuel pump, the injectors, fuel rail and its sensors then remove the fuel lines and tank for professional cleaning.
In you case it appears the garage concerned did not follow the strict conditions that are necessary when doing this job. Sadly if any part is neglected (in your case apparently the tankl) then it is highly likely any repalcement parts may again have been contaminated and you could be back to where you started.
Armed with this knowledge I suggest you insist the garage do the job again or compensate you acccordingly.
Thanks for your reply,it's much appreciated!
The more I know, the more confident I'll be about talking to the garage.
We paid £2500
Here's what was done to the vehicle originally:
Found massive amounts of filings in the fuel tank, so removed the CR pump and examine, beyond repair.
Remove injectors, test. Strip and service, reassemble and test.
Drain fuel tank, remove tank and clean, also clean all fuel lines, replace fuel filter, supply clean diesel, reassemble vehicle and road test.
New timing belt kit, fuel filter, diesel system treatment, Delphi control valve, CR washer sets, CR nozzle kits, CR pump, glow plugs, fuel tank seal.
Diesel labour alone was £600.
Is this Tank sender unit something that should have been replaced at the time? It just seems strange that it's coming up with exactly the same fault again? Although they're saying it's not connected in anyway.
Also, why would air be getting into the fuel lines? I think that's what they said, can't be totally sure, but know for sure they mentioned the Tank sender unit.