Good morning, there are many different models of Minerva tap.
If the control knob is lose it should have a plastic bung in the centre which can be remove using a large flat object such as a small screw driver or knife.
Once this bung has been removed you should see a small Philips head screw in side the hole the bung was covering.
Generally a half turn of this screw should be sufficient to tighten the knob, once the screw is tight, use the tap and make sure your happy it is freely moving, then replace the bung.
Please let me know if you have any further queries, if you have difficulty locating the bung, please upload a photo or advise of the model of tap.
Good afternoon, do you still require assistance?
Just wanted to follow up to see how you require any further assistance?