UPDATE 1-UK shuts down wind turbines as storm Katia hits

* National Grid stopped 750 MW of wind capacity on Saturday night

* Stopped 300 MW of wind capacity on Sunday night

* Heavy winds expected Monday afternoon, evening - Met Office

LONDON, Sept 12 (Reuters) - Britain's energy network operator National Grid may need to shut down a number of wind power farms on Monday night as strong winds threaten to overload the transmission system with electricity at times when demand is low, a spokesman said.

"If we've got constraints and too much generation we'll go back in there. Nothing (is decided) yet today but if it stays this windy, we may have to look at it," he said.

The grid operator had to shut down 750 megawatts (MW) of wind power capacity in Scotland on Saturday night and 300 MW on Sunday night as the network was congested.

Heavy winds are expected to hit Northern Ireland, Scotland and northern England on Monday afternoon and evening as post-tropical storm Katia brings gales and heavy rain to the region, Britain's Met Office said in its latest weather forecast.

"All parts of the UK will see gusty conditions through the start of the week, with the highest wind speeds of 75 to 80 miles per hour expected over northern and western regions of the country," the Met Office said.

Scottish and Southern Energy , which operates 840 MW of wind power generation in the UK and Ireland, refused to comment on whether any of its Scottish wind farms had been switched off over the weekend.