UPDATE - PROBLEM!

Over the last day or so, since having new top mounts fitted, i have been experiencing a problem that has been getting progressively worse. I have been getting a grinding/droning noise that doesn't happen under acceleration, only when i take my foot off the accelerator. I can feel this through the steering wheel too. If I apply the brakes, the noise and grinding seem to go away.
I'll be taking my car back to the garage that did the work this morning, as it has only happened since then, but i'd like to show up informed, as this is likely to be a symptom of the work they did, and not just some wild coincidence which i'm sure they'll try saying!
Does anyone have an idea what this could be? How much of the strut assembly would they have to remove to get the top mounts in?