I am doing a kitchen refurb and intend to change the cupboard doors whilst retaining the carcass. The existing doors are non standard; presumably an imperial size : 19.5"x23". I am trying to locate a supplier of this size or a non expensive manufacturer.
can anyone help?

That would appear to be a standard 500mm door? they actually measure 497mm wide?

doorsize 496mm wide 585mm high . 496mm sems to be a standard width . know any fast reliable suppliers?

Screwfix, B&Q sell individual doors

is it much cheaper to just replace doors as it is to replace whole units?

A lot depends on how much you are prepared to pay.
Have you tried Howdens ? http://www.above-it-all.co.uk/howdens/about.htm

My first search on Google, found this

http://www.kitchenfacades.com/