ATC SCM19 V2 Recommendations.

I'd fill the stands with dry play sand.  It will provide a less resonant foundation, and less easy to tip.

@kb54 

The stands are filled with something heavy already. Each one weighs about 25lb.

@tony1954 

Whatever sound the stands make, that is added to the speaker.  Mass is your friend in stands so the fact they are filled an don't resonate is good.  42" away sounds good.  Now how far from side walls?

So you are a Brit too!  Well if you ever go back, call them a week or two ahead and get an ATC factory tour.  It's fascinating because it's the way all the speaker makers used to look (winding coils, building drivers, etc ) but almost none look like that now.    Most buy all the parts/cabinets from a third party.  

+1 on the Sound Anchor recommendation, I used them for JBL and Westlake monitors. I still have a 3 leg pair for some vintage JBL’s. What is nice is that you can attain an exact height, and exact footprint for your speaker cabinet. Warning , do not use the blue dots as they will decay and release oils/chemicals into your speaker cabinets. Also their black finish ,while attractive is subject to wear from handling as they mix a course substance into the finish. Cheers , Mike. 

@lonemountain 

Side walls aren't the same.

On the right side the distance is 36" - 42" depending on how it sounds. The right side is all patio doors and windows.

The left side is around 8' to the adjacent wall and then opens into the kitchen.

Haven't been back to England yet, but it is on the bucket list. The last 4 times to Europe have all involved wine tours that start and stop in Paris.