I certainly would start by checking glow plugs first. if ok Do a leak off test of injectors
you most likely will find one of the injectors has a leak off a most common fault on these 1.5 renault dci's which cause cold start problems. I think your garage is clutchimg at straws looking to replace all four injectors. Just ask your your garage have they done
a leak off test Checked operation of glow plugs starter operation cold Battery check
Its common and good practice to replace all four injectors but the price is very high look on ebay.de or try and find an injector firm that reconditions them more likely to find in Germany than Luxembourg.
Had a look on German ebay a few firms selling them Artikelnummer: 230516530932 around a €130 to 190 each another one selling new parts Artikelnummer: 230553390593