Colin Wilson wrote

Can anyone give me an idea of the typical measurements of an install
where the soil pipe is embedded in concrete in the floor - ie. what the
normal distance away from a wall the centre of the soil pipe would be ?

There's no standard dimension Colin. The average is about 200mm off the back
wall, so you'll have a bit to make up, using a ? connector or maybe a board or
copper pipe spacers behind the cistern. Use a low level suite with a flush pipe
(ie not close coupled) as this allows more adjustment, but I agree with Rick you
should consider the toilet seat. You may find yours will lean back a long way.

Most manufacturers put dimensions on their website if you know where to look -
usually under "trade" or "specifier" etc. This is an example of a low level
suite from the Twyfords website:
http://www.twyfordbathrooms.com/twy_.../ChantalLL.htm

I'm a bit worried about the sideways measurement. 29cm is far too small - you
wont be able to sit down! ) 40cm is the basic minimum and 50cm is
comfortable. This needs checking maybe?

HTH
Peter