After a friend at work tried some coffee I ground for him and it took 3 minutes to extract a shot it wasn't hard to work out something was wrong with one of the machines

I fitted the blind and ran the pump for 30 seconds, the OPV doesn't open during the 30 seconds and I get 13ml when its shut off so it must be way over pressure.

What affect does this much pressure have on taste?

First of all, see I still don't get it: Why adjust the OPV? for background information. To answer your last question, telltale signs of excessive brew pressure are bitterness and dark crema / oil droplets, i.e., overextraction. If the puck channels, then add the symptoms of underextraction, i.e., dull flavor, sourness, light crema color.

My results are starting to make more sense.

Well like an idiot I was measuring from the drip tray pipe.

Now that I've done it correctly with a bit of tweaking to the OPV I'm getting 100ml in 30 seconds through the return pipe.

Not sure how much it was over from the factory as when I noticed my blunder I'd already got the socket on the OPV so just carried on. It was definitely over pressure as I unwound it at least one turn. Will be interesting to see what the morning espresso is like.