The cheapest quality new MTB is a Voodoo Bizango. It is well spec'ed and made for the money and gets 10/10 in most reviews but is not a lightweight xc bike. As the bikes are supplied by
Halfords you need to give them a good once over to make sure all is OK but it is unlikely you will get any issues that a quick tweak won't fix. Check the web for reviews.
To get more for you money second hand is a good choice. Bikes like the Specialized Carve / Crave are well spec'ed entry level XC race bikes. Just be careful they are in good condition and the 2013 or newer models have much lighter frames and wheels. Plenty of other similar bikes from other manufacturers available. Also if you decide to sell afterwards you should get most of your money back.
Thanks for the advice. My purchasing follows a familiar pattern, I buy to hold whether it's gear, property or shares - I have never sold anything on (except shares).
I'm in love with Cinelli, and unlikely to be smitten by a MTB brand because it speaks to all the qualities I do not value - recklessness, adrenaline, childishness. Of course, i have all these characteristics, i just like to pretend I'm above all that.
The Geo is a bit pricey for something I don't want, I could've sold myself a CX bike as "one for the Mrs" but I can't race on it...so I might go with the Bianchi as it looks cool.
I suppose I could make it a bike for my boy, especially with a ridiculous name like "Bizango" but Indont know if I could stomach housing that next to Cinelli, Titleist, Vega, Porsche, Nordica.
Mind you I'm reduced to drinking "Faustino" tonight.
So all in all, it turns out I'm a bit of a snob!