wrote:
Within the last year we moved house into a newish build (circa 96).

In the lounge there is a vent grill near the skirting, which
corresponds with a matching grill on the outside wall. In the summer we
didn't notice it, but now it's getting colder we can feel a massive
cold draught being generated by it.

Why do such vents exist? It is to prevent damp problems?

Can we cover it over in the winter? Surely it will send our heating
bill through the roof if we don't!

Many thanks

If there is a gas appliance in the room it might be for ventilation for
that, in which case it could be dangerous to block it up. If there is no gas
appliance in the room it should be safe to block it up. Air bricks which
ventilate the space below wooden floors should not be blocked up otherwise
the joists may become damp and then rot.