March 8 (Reuters) - British spot gas prices fell on Thursday as strong output from mid-range storage sites contributed to an oversupplied system.
* Within-day gas was down 0.6 pence at 51 pence per therm (p/therm) by 0958 GMT.
* Day-ahead gas rose by 0.4 pence to 51.50 p/therm, reflecting the chance of cooler weather and low stored reserves.
* The system was oversupplied by 11.8 million cubic metres (mcm) early on Thursday, National Grid data showed, with demand at 312.8 mcm/day and supply at 324.6 mcm/day.
* Expected daily withdrawal from British storage sites was at 35 mcm as residential and commercial demand weakened and UK Continental Shelf output was expected to improve, reinforcing bearish sentiment.
* A fresh liquefied natural gas (LNG) delivery from Qatar is due to arrive at South Hook in Milford Haven, Wales, on March 24, signalling a pick-up in shipments on top of the two tankers already in-bound.
* An unplanned reactor outage at Heysham 2-8 helped to support day-ahead baseload UK power prices, up 2 pounds to 53 pounds per megawatt-hour (MWh).
* Norwegian flows to the UK rose by 10 mcm/day to 130mcm/day, figures from Norwegian pipeline operator Gassco showed.
* The April contract rose by 0.39 pence to 48.47 p/therm.
* Day-ahead gas at the Dutch TTF hub was unchanged at 18.80 euros/MWh.
* The benchmark Dec-18 EU carbon contract edged 0.08 euros higher to 10.72 euros a tonne.
* Thomson Reuters analyst view: here. (Reporting by Oleg Vukmanovic Editing by David Goodman)