UK GAS-Prices firm in near balanced market

* System 2 mcm long

* Centrica (LSE: CNA.L - news) delays Rough move to single train injection

LONDON, Aug 21 (Reuters) - Prompt natural gas prices in Britain were firm on Friday in a tight system with supply and demand closely matched.

Prices for within-day delivery were unchanged at 38 pence per therm at 0917 GMT.

Britain's gas system was slightly oversupplied by 2 million cubic metres (mcm). Demand was forecast at 190 mcm while flows were expected at 192 mcm/day, according to National Grid (LSE: NG.L - news) data.

"Prompt prices have firmed in line with tighter intra-day supplies than seen through the week, together with potentially higher Rough demand over the weekend with the switch to single train injections further delayed," said Marcel Boonaert, head of trading and portfolio at Wingas UK.

Centrica on Friday delayed Britain's Rough storage site's shift to single train injection until Aug. 22-25.

Gas for September delivery was up 0.33 pence at 38.85 pence/therm.

"Upward moves are also being seen on prices across the curve, taking direction from a further depreciation in the value of sterling," Boonaert added.

A weaker sterling rate is bullish for UK contracts.

Prices were unchanged on the Dutch TTF gas market, with day-ahead TTF gas trading at 19.15 euros per megawatt hour.

In Europe's carbon market, benchmark EU Allowances (EUA) were 0.07 euros lower to 8.30 euros a tonne. (Reporting by Sarah McFarlane; Editing by David Evans)