OK. Time for an update.
I connected the A5000, Model B and clock box all up with a known good T-piece and still they cannot talk to each other.
I swapped over the Econet modules in the Master downstairs and the A5000, which still didn't work.
I connected them all back up, using the new run of cable, etc. and fired up Level 2 fileserver on the Master, and the Model B can see it and logion!! Finally a result that tells me something - the Model B works, the Master works, the econet module in it is the one from the A5000, so presumably both of those work. And, the new cable run works! But the A5000 can't see the FS on the Master, and still neither can log into the L4 on the A5000. Drat.
So... it must be something in the A5000 that's causing me problems other than the econet module, which given the buffers on the module blow first if there is a line issue, means it most likely is a software problem.
Of course all this power cycling of the A5000 has thrown up more issues; the poor thing has two elderly hard discs in it, and the second one seems to be dying - I'm getting all sorts of strange transfer errors on power up, even before the "RISC OS / Acorn ADFS" banner appears. One thing after another ...
OK then, let's go solid state: I try to fit an SD-IDE adapter I got off Mark of retroclinic when we were at Byte Back and after some hunting for hform, got it formatting a 2GB SD memory card. But it keeps failing. A quick email to Mark and he reminds me that there's a 512MB limit on RISC OS 3.1 .. argh. I'd forgotten all about that one; used to hit it all the time with DOS machines way back when... (At least it didn't do what one client machine with a caching IDE controller did to me once - let me initialise a 1GB disc, and as we were copying data to it, as we hit the 512MB, looped back to the start of the drive and trashed the partition tables and directory...)
So, rather than limit the format and waste 75% of the 2GB card, I've found some 512MB mini-SD cards on eBay for 99p with free delivery, so ordered one of those. I've disabled the second IDE drive in the A5000 for now, as there wasn't anything on it I need at the moment, and I hope I can copy everything off it when I get the new card. I've got master/slave issues to work out on that yet ...
And then, once the A5000 is all up and running on the new drive, I can try and find out why it's not liking the econet any more..