Thanks guys - pretty useful, as I actually couldn't get my head around how it would work. Clearly not right to completely reset the odometer, as the rest of the car has done a few miles, but somehow needs to register that the engine isn't as old as it may appear...
Not looking forward to having to tootle around like Miss Daisy for a while, but at least it's a new engine (well, will be when it arrives)!
Currently looking like a minimum of a 2 week wait - and patience isn't something I'm blessed with...!!!