Things are looking up! The fab house had no problem making the donut shaped PCB and I’ve just received the first batch.
And what’s more, they actually fit in the saucer!
The next job is to build one up and make sure it still fits with all the components added – that’s still a gamble. I’ve gone for surface mount resistors to save space, but after checking and checking, the one thing I didn’t notice until just after I sent the design off is that I’ve used the smaller 0805 package rather than the intended 1206 package which means they will be even more fiddly to hand solder.