Bit of bike buying advice

Just to preface this I have read the guides on here but they are all out of date with mostly bikes that don't exist anymore or have been updated. 
Anyway I'm looking to retire my ancient mountain bike of around 15 years for something a little more up to date. Budget around £300. I have read things like the Rockrider 520 are good for the money but the nearest Decathlon is over an hours drive away. Could get one delivered but I'm not sure about size? (I'm about 5' 11"). http://www.decathlon.co.uk/rockrider-520-mountain-bike-id_8293188.html
I've seen plenty of other options in the price range such as this Schwinn for £280. Any good? I'm not too sure what to be looking out for other than hydraulic disc brakes http://www.wiggle.co.uk/schwinn-rocket-3-2015/
Any help much appreciated!

Anyway I'm looking to retire my ancient mountain bike of around 15 years for something a little more up to date. Budget around £300. I have read things like the Rockrider 520 are good for the money but the nearest Decathlon is over an hours drive away. Could get one delivered but I'm not sure about size? (I'm about 5' 11"). http://www.decathlon.co.uk/rockrider-520-mountain-bike-id_8293188.html
I've seen plenty of other options in the price range such as this Schwinn for £280. Any good? I'm not too sure what to be looking out for other than hydraulic disc brakes http://www.wiggle.co.uk/schwinn-rocket-3-2015/
Any help much appreciated!

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Look at the Calibre Two.Two or Point.5. Both slightly over your budget but they have had sales, the point.5 is currently reduced and a bargain, as is the Rockrider 560 which is again over your budget but for an extra £80-£100 you get a lot more bike.
I'm 5'11" and find the 18" Calibre Two.Two perfect.
Bird Aeris 120 | Boardman Team 650b | Boardman Pro FS | Calibre Two.two
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I'd rather stay closer to £300 than £400 as like I say it won't get too stressed, just light pedalling about through the woods etc.