thanks bt bert, I am updating this hopefully for other peoples benefit in the future...
the second BT guy came yesterday, I actually had tried to cancel it as I was not getting an errors for a couple of days,,,but it was too late to cancel and I thought I was doomed to face another possible £200 charge and to be told there was "nothing wrong".
Anyway I plugged in my landline phone as I was trying to call Xilo to cancel the sfi...for some reason I tested the phone was plugged in by calling it from my mobile, it rang and I went back to my PC to find xilos number (i should have it on speed dial by now

) anyway! the net was down..i thought thats odd and when it came back up I tried ringing my landline again...i watched on the router stats page and the crc errors were flying up...right until the second I stop ringing the landline...so i thought it must be a filter or phone problem. I swapped the filter and tried again...exactly the same thing...no errors to start..then whilst ringing errors were flying up and theyd stop dead when i stopped ringing.
Anyway so I thought it must be the phone itself, unplugged it completly from the master socket so only the braodband was in, exactly the same thing happened. I even got my friend to phone me several times, and sure enough without fail it happened each and every time (if i picked up the phone and talked, no problem at all, errors stopped dead...its literally just the ringing thats been causing all the problems somehow..)
Shortly after, the BT guy turned up (over an hour earlier than predicted!) I showed him this happening on my pc, he plugged in his own equipment, exactly the same thing happened. This was great and the first progress or sign of a pattern I've had since this all began months ago.
He re-did the wiring at the top and bottom of the pole, and the green box, and even went to the exchange to check it out, but nothing fixed it.
To cut an already long story short I should be getting rein people to investigate this now, and Im hopeful I wont be facing the £400 charges now. It was just pure luck that I found this out just before he arrived. So anyone who is having similar symptoms trying ringing your landline even with no phone plugged in and see if you get the same thing!