So, a few people recommended Rigida Spunik rims, and as we DNSed The Dean for weather reasons anyway, there was time before Yr Elenydd to send the hub to a wheel builder. So I thought, anyway. After being told a week's turnaround, and sending the hub away two and a half weeks before the ride date, nothing. Still nothing three weeks after we sent the hub away.
I ended up sourcing another wheel at short notice, a 48H deep section rim from JD Tandems.
Once I get back the 40H hub (with or without a wheel built onto it) I'll look at what to build onto that and whether to build a new rim onto the front hub also. The wheel builder originally contacted could have had more work out of us but doesn't seem very reliable.