If your meter shows video levels then theres half a chance it will show sync pulses. It will probably overlay a squiggly line on the display, and you need 1 volt peak to peak on this squiggle...

ground loop isolator.

Could be that adi. What does the picture look like connected straight through to the dvrs monitor?

but I just need to know why I can see image fine in the office with the balun plugged into my handheld monitor but the same balun plugged into the dvr gives an un-useable image.

Simple, a monitor is much more tolerant of your poor quality video signal than your dvr.

Try a test camera over the 150m balun run to see if its any better than the wireless link

Simple, a monitor is much more tolerant of your poor quality video signal than your dvr.

Exactly.

What P2P or Sync values does your tester say?

Thank you, I had already stumbled on the idea of ground loop inteference and have ordered a couple of filters, if thats not it they'll be useful to have anyway.  I'll be trying my colorbar generator over the balun to the dvr when I go back next week to have a look at the cat5 link and try the level meter.  I was wondering on the possibility of the signal being too strong rather than too weak aswell and whether a 75ohm termination may be needed.  The picture on the dvr was intermittent between no signal and a very grey skewed and fuzzy image.  Thinking about the potential of ground loop interference, the yagi aerial on the reciever is attatched to a metal pole attatched to the tin shed, which would be a source of grounding, I may try insulating the pole from the bracket aswell.

It's better to remove a ground loop problem at source whenever possible.

If the levels were too high I would expect the picture to be stable but lack quality. Althought this may depend on the DVR.

I know some units will crash/freeze intermittantly if the signal is too hot.

Eithier way the best thing to do is check with a SLM at the DVR.

Your baluns should be isolating any grounding issues. As you have said is break it down and see where the problem is first

Not nessacerely (however its spelt)

... and thats from experience.