In reply to cfer:
> Wheels would be great but with only £150 I'm not sure I would feel the benefit as much as other parts
Slight tangent, but I was looking at upgrading the wheels on my Cannondale Synapse and found that for disc compatible wheels there was a very limited choice, and either less than £150 a pair, or over £750. Hopefully more will come out in the next year or so.
Having spare wheels is always useful, though possibly less so if you have both a road and a CX bike. Other than that, stock up on spare tyres, chains, cassette etc.