Headset play
 

If I rock my MTB at standstill with front brakes on there is play in the headset.

Do I need to replace the whole thing? How do I determine the specs for a new one? Its a 1 1/8th Aheadset.

Thanks

Maybe not, you'll need to do a little more figuring before you can make that decision.

Check out the Fork keeps developing play thread. It sounds like it is tracking with information you can use to troubleshoot your issue.

Good info. I can feel play between the top of headtube and top race (if that correct terminology). Everything is tightened OK. It has started to creak a little recently.

That sounds like a loose headset. It depends on whether you have a threaded or threadless headset as to what to do next. I think Aheadset only does threadless though.

Possible cause
 

When replacing the stem would tightening the stem bolts before headset cap make any difference if the stem wasn't seated against the spacers properly?

Yes. You need to adjust the bearings ("preload") with the cap before tightening the stem. Look on the Park Tools website for a good "how to" on threadless headset adjustment.

I was noticing the same thing on my old bike, took it apart and the bearings had rusted, I needed new ones, but yours could just be loose.

Take it apart and check the condition of the bearings and cups or sealed bearings...the comment by Camilo is spot on!

It definitely made a difference (if not fixed it) when I refitted stem properly as per Camilo. I have also a noticed creaking noise so have picked up a new CC S8 to replace existing headset. Thanks for advice.

Yeah, the creaking is probably more likely from the bar/stem interface. If it's actually the headset, it's likely just due to oxidation between the headset press fittings and the frame. Just greasing them would take care of the problem, but you have to remove the headset to do that anyway. Given that operation for a home mechanic, it's probably worth it to throw a new headset in anyway :)