for the cost of a bespoke kitchen we could build an extension to our house and have a cheapo kitchen fitted that was twice the size

so the first question that's been asked (and answered in a previous post by stu) is "which room are you going to put the kitchen in?"

you also have to factor in things like stripping out the old stuff, electrical compliance, plumbing and gas safe registered folk to get everything legal and certified

ikea do a great kitchen planner that should give you some idea of what will fit where and the relative cost, but a nornal unit from the likes of b&q will probably work out a 1/4 of the price of a custom made unit from the experience we've had, and we've been researching this for about 18 months and still haven't decided