There was no knock before and there should be no knock now with just a spring change especially in your car with only 65000 kms and normal usage I assume. Most owners will never change a front strut up to 100000 kms unless they do an upgrade.
The strut mount bushes should be OK - fairly low Kms - and you had no noise before the spring change so they should be fine now also with the same strut re-installed.
Sounds more likely that there is movement somewhere because in the spring install (where the entire strut assembly has to be removed from your car) a nut has not been torqued up correctly with the re-installation of the strut assembly. One area you should check is the sway bar links on the noisy side where one end of the sway bar link is bolted to the front strut because if that is loose you will get the knock you are hearing. The sway bar links themselves should still have plenty of life in them at 65000 kms but when worn they will clunk on the ball joint type end fitting where the wear occurs.
If the guys you took the car to were half competent, rather than doing the so typical fobbing you off exercise, they would have been able to diagnose the source of the noise and they should also have checked their work thoroughly. But they don't sound competent so I would take the car to a suspension specialist and get them to check it over for you. You had no noise before and now you do so they more than likely screwed up somewhere.
Lots of people here have done a simple spring change just as you have done without new noise issues originating from the front suspension.
