You should be able to screw the floorboards down into the beams underneath. It is amazing what a difference this makes. I did this when they were changing carpets in my parents place, and it really stopped the creaking.
In the US, at Home Depot, they used to sell a fairly inexpensive kit that allowed you to put the screws even through carpet. You may want to check the usual suspects (e.g., B&Q, Woodies (and no I'm not affliated with either!) to see if they have something like this here.
Best,
S