I rendering a 250+ frames scene to images in P7. After approx 12 renders, I lose the lovely reflection on the kitchen worktop, it just becomes a matt surface. I have no idea why it does this, I'm thinking a memory issue but I have 6Gb in this machine. Any ideas?

You haven't accidentally animated your worktop material, I suppose? 

Quote - You haven't accidentally animated your worktop material, I suppose? 

I don't think so
For example frame 171 has the correct reflection & 172 has no reflection. If i go back and try and render frame 171 I now get the incorrect no reflection.

 You haven't accidentally animated your worktop material, I suppose? 

or animated the lights ?

Good point with the lights, they didn't seem to be animated but the animation box was ticked. I unticked and tried again, same thing happened.

By animating the worktop do you mean adding a movie to the texture or a proceedure map? The image for the worktop is the default material that came with the figure. Thats the one I'm using but otherwise I don't know how to check if its animated.

I would open the keyframe editor and look at the line for the countertop, it should have no keyframes. If there is one not at frame 1 expand it to see what parameter is keyframed.