LONDON, Dec 7 (Reuters) - British prompt gas prices gained slightly on Friday but the system was slightly long as milder temperatures reduced demand, offseting the impact of lower supply from some key pipelines.

British day-ahead gas prices gained by 0.33 pence per therm between Thursday and Friday morning, trading around 69.53 pence at 0900 GMT, while prices for within-day delivery were unchanged at 69.50 pence per therm.

Earlier this week prompt gas price hit a 10-month high of 71.25 pence/therm as Britain experienced colder weather, including sub-zero temperatures in many areas and snowfall.

Gas demand on Thursday was expected to be 339.4 million cubic metres (mcm), 24 percent higher than the seasonal norm, according to data from National Grid, while flows seen at 342 mcm as supplies, down from Thursday.

Analysts said the outlook for prompt gas contracts was 'sideways to bearish,' although increased supply from storage and some pipelines was offset by disruptions to others.

'The impact that the scheduled maintenance at (parts of) the Norwegian Continental Shelf could have on flows to the UK remains a risk factor limiting the downside of the contract,' Reuters Point Carbon said in a daily report.

It added: 'But higher storage withdrawals and supply (of liquefied natural gas) from Grain (LNG Terminal) could provide flexibility if needed.'

Daytime temperatures are expected to become slightly milder on Friday, Britain's Met Office said, adding that temperatures would reach a maximum of 7 degrees Celsius in southern Britain over the next few days while night time temperatures would most likely remain above freezing.

Further long the price curve, the summer 2013 gas contract was valued at 62.60 pence/therm, unchanged from Thursday's close as crude oil prices were broadly steady compared with last night's settlement, trading at around $107/barrel.

In the power markets the contract for baseload (24 hours) delivery gained 20 cents, trading at 52.35 euros/MWh, relatively unmoved despite an unplanned outage on Friday at EDF Energy's 660-MW Heysham 2 unit 7 reactor.

The reactor came offline at 0930 GMT on Friday, EDF Energy said in a statement, and had only returned to the grid earlier in December following turbine repair works.

(Reporting by John McGarrity, editing by William Hardy)

(john.mcgarrity@thomsonreuters.com)

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