Having a bit of bother with the main water in to a barn conversion. I think the leak is from the stopcock - not from the compression joints but where the tap section (sorry don’t know technical term!) meets the main bit. See the picture where my finger is pointing. 
 

is that a likely place for a leak (stopcock is brand new)? I’ve tried tightening it but not sure I can go any more but there still seems to be a little leak. Any suggestions?

There should be a fibre washer at that join. If you can turn the water off at the road you could take it off and use a jointing paste on it. (Others will recommend what’s best to use).