thank you both, each option has it's advantages Google SketchUP seems to be a free application (not the 'pro' version) but I don't want the sketch in 3D and the learning curve looks a little steep for making a single drawing.
As for Omnigraffle looks good, but if I am going to pay for software, I would like a disc and not just a download key (old fashioned like that), but the Apple store (uk) only sells the pro version. Perhaps I can get the next level down from another retailer.
Thanks guys, each are good options.
Now I have to decide if it is worth buying software for a one off drawing.
Thanks to you both.