Time Trial / Time Trialling / TT

To that end (and I realise that fitness and experience are both going to be bringing me much more substantial gains) I have earmarked the planet x 82/101 wheels as my next purchase, is this the best use of the cash in buying speed or would a professional bike fit and a aero helmet be a better option?

Definitely get the aero helmet first, much better VFM. Then a skinsuit, then wheels. Or wheels first, cos they look and sound fast :) Play around with your position as well, Adam Tophan's book is pretty good at explaining how to get a decent position and is a very good general guide to TT'ing IMO.

I'm only in my second season, and was doing similar times to you last year. Unfortunately I've bought pretty much all I can justify in terms of aero, so I can only go faster through training now :(