Jez/Potts - like the two of you, I'm about to buy a Dac at around the £500 mark and I'm pretty much sold on the Rega (seems like the natural choice with my all Rega set up, aside from anything else). Though, the limitations of the USB input (i.e., that it's not asynchronous and limited to 16bit/48 Khz files) is causing me to hesitate.
Obviously, if you're intending to play a lot of high-res files via the USB, the Rega probably isn't the best option. Adding an asynchronous USB convertor is a possible solution. Although, I believe the Musical Fidelity V-link is itself limited to 24bit/96 Khz files. Whilst 24/96 seems to be the preferred resolution for most high-res music available today, I would want an element of future-proofing and this would therefore rule out the V-Link for me (at least in it's current form). I'd personally probably opt for something like the M2Tech Hiface convertor. Though, the price of the Rega plus Hiface takes you pretty close to the price at which the M2Tech Young Dac was recently being offered at here in the UK.