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For commuting, MTB and a bit of road too.
Blackburn mars 3.0
awsumz for the price
http://www.planet-x-bikes.co.uk/i/q/LISMRTR2-2x1/2-RED/smart_lunar_r2_rear_light___2_x_1_2_watt_led___red_black
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Another vote for the Blackburn Mars 3.
£4 from Tesco - http://www.tesco.com/direct/activequipment-3-function-led-rear-bike-light/212-3847.prd?pageLevel=&skuId=212-3847
If you feel the need to spend £25 you could buy 6 of these.
I've got some of the phart lights from on-one. £4 each and mega bright.
Not sure why you need a rear light for MTBing though? It very distracting for following riders on the trail IMO.
I'd spend a tiny bit more and get an Exposure Flare. They're painfully bright. Indeed I pity those behind me.
Get 20 from poundland with enough batteries to run them for next two years.