ABS / Grinding Noise at Rolling Speed

[trisnain]

So after having new front arms fitted yesterday my car now makes a grinding noise when you apply the brakes softly at rolling speed, the brake pedal also judders and sinks a little as if the ABS is kicking in. This has never happened before, present as soon as I left the garage. I took it back first thing this morning and they checked everything they'd fitted to make sure everything was tightened correctly. They hooked it up to the diagnostics which showed a few errors for the AirCon but nothing else at all. They put it on the brake testing machine which was fine and oddly we could not emulate the symptom when on the rollers. I've left it with them for the day, but they are suggesting it is nothing to do with them - Which I find an incredibly massive co-incidence if not so. Any suggestions? My guess with limited knowledge is that they may have knocked an ABS sensor while working on the arms and this is causing hassle - but then wouldn't that throw up a fault code? Confused... any help appreciated before I go back after work, the motor is driving me nuts at the moment.