Blimey, gone outside in the cold with me tape measure especially for you mate

Very difficult to measure without knowing exactly what you're after but...
Space between load cover rails approx 49" or 1m25cm
Front to back approx 40"-44" or 100-125cm usable space.
Height @ opening 31" or 80cm and 34" 87cm further in.
Don't take these figures as read, they are very approximate. i.e. there is a quite a usable width in front of the arches and a good degree of height, unlike V70 volvos etc.
The load bay is completely flat with no lip, the wheel arches don't intrude too much like some cars and the tailgate is flatter and less raked than most other estates (406, 5 series, A6 etc) The omega estate will also accept loads up to 3m long, wood, piping etc.
You could always get an omega hearse if you were after more space! 8-)
I would try and pick up a brochure off ebay which shows dimensions really well. Go and have a nosey round at a dealers which should confirm other things, otherwise, post your questions up here.
