Thank you both - very useful.
Particularly your dimensions warning Colin. For some reason I had it in my head that they were equivalent(ish) in size to the 33s.
No issue with the shelving unit taking the weight - it is built in and very strong. And I have some headroom available, but I will clearly have to check if it is enough.
I wonder how 3100s would cope with being 'boxed in' - ie. virtually filling the height of a shelving unit 'alcove' and, potentially, having books stacked against them...!
(I know, I know. I promise it isn't my most critical listening room.....!).

Neil