Tim B, thanks for your advice. I have spoken to Paul Hewitt and was impressed by his very knowledgable yet easy-going approach. He builds a touring wheel that takes both 28 and 32mm tyres. I will be buying from him whether it's just a frame (+wheels) or the complete machine as getting a proper fit is very important to me.
Apologies to all you Thorn/Mercian fans out there, but I'm leaning towards the V.N. Amazon at present
You also make an interesting point that when one is trying to combine an audax and a tourer in one machine, titanium has a lot going for it. As well as speaking to Paul H, I've been to see Paul Smith at Bikeplus and he basically says more or less the same thing. He rides a Yukon, which is ideal for credit card touring but not great as a "load carrier".
Would appreciate hearing from any Amazon owners out there.