Strange this gets mentioned today, I noticed that NETTO have them, well they were advertising them for £15 on the leaflet they stuck through my letterbox last night.
Other places I'd look are places like Yoemans/Milletts/FAS.
A quick google on 'Work wear' brought up places such as
http://www.kelticclothing.co.uk/ which do a range of stuff.
please note other clothing and work wear suppliers are available.