Hi,
The 11.77mm looks about right for the ones I got from C and Ds.
As for the cases for £150, then it depends on how good they are. If they are in good clean condition then that is not a bad price but if they are not much better than the ones you have then it may be best to sort the ones you have.
Have a really good look at them and compare them to yours. Check all the threads and joint faces. Look to see if bearings have been loose and/or rotating in the case. If you are happy with them offer him a bit less.
Dave