Mike knows you, and knows your position. I'm pretty sure that if you phoned up him or mailed him, and said "I need to buy a frame on the cheap, can you tell me what sort of size I'm looking at for brands X, Y & Z?", he'd help you out. From there, you'll just have to work on your position and see what feels right. The Old School method is to set up a mirror in front of the turbo - but you could also get Heather to video you from front, side and rear. I'm sure we'd all be happy to offer constructive advice!
And as I said elsewhere, keep your eye on the For Sale section on TTF - full bikes and frames often go very cheaply on there. eBay's another option. This is the best time to buy 2nd hand cheap, as it's the end of the season so prices will fall.
Alternatively, £500 buys you a brand new PX Exocet 2 frame and fork, £1k's worth of kit. Could do a *lot* worse than that. Should be able to source a new groupset for less than £300 (eg 105), then you'd need wheels (reuse road bike wheels for now?) saddle, and base bar/aero bars. 6 months to the start of the season - nice way to spread out the expense and build it up over winter.