What's the best lock I can buy in Manchester tomorrow?

I constantly hear shop staff giving bad security advice and selling anything they have. I got really annoyed with one yesterday in a popular chain store who was useless with a customer and had obviously never locked a bike up in his life.

big stores don't always always train their people about bike security and some who work there obviously don't know a great deal about bikes.

most cable locks can be snipped in a few seconds .

any D lock should be at least 16mm shackle . fill the space with as much bike as possible. stops bottle jacks

same applies for the chain which should have links which have a diametre of 16mm.
keep the chain of the ground.

most chains in bike shops are not 16mm diametre as the weight is simply tooo heavy.