To original poster: your ship has come in. Check out the new dCS Puccini u-clock which is both a separate word clock and asynchronous USB to PCM converter. For your setup it is probably the best device on the market, and if I understand correctly it would be a double upgrade for you. First, it is an external wordclock for both your Purcell and Delius AND your transport which in this case would be a USB capable computer. Second, and I am not entirely sure about this, but using proper asyncronous USB has some inherent advantages with respect to jitter over SPDIF and so it may be better than what you are using now in that respect.
Also the price is expensive but not too bad--about $5,000?
Also the price is expensive but not too bad--about $5,000?