Is Simon at the exchange still mastering?
When I was cutting lacquers and acetate masters at scrunch recordings in Philadelphia, all the kats with budgets or clout went to Simon for mastering.
Couple of tips from my days in the 'loop':
-If you can, don't limit or (god forbid) normalize your mixes.
Mastering engineers need a little headroom to play with.
-make sure you monitor with a sub. Everything below 80 cycles becomes very important for D'n'B and if its at all mud, you get mud.
-When mixing focus on the drums first, bass second everything else third.
The few tracks I actually produced, I would extreme HP all the drums to give more space for the bass. You can also sidechain the kick to a compressor on the bass. That helps to make a little space in the lowend.
-someone mentioned downsampling. That is a great trick, so is using dynamics to effect. Those splat snares take some tweaking to get.
-One of things I grew to LOATHE after that job was the production value of reason/acid/fruityloops. If at all possible, stay out of purely digital.
Outboard samplers are key to good drum and bass...an old moog helps too.
-If you are producing D'n'B, dont be shy to use multiple bass lines.
I will disclaim these tips with the fact that I haven't been working in the scene for many years now...
Hope something in this post help!