A few bits an pieces sorted out.
TalkTalk finally fixed the aDSL line.
So I complained bitterly, obtained some refundage and then switched out to Post Office broadband - it looks like the best value contract and given that it's for failover, should be sufficient. I honestly don't think it will be any better than TalkTalk should things go pear-shaped, but I can quite justify the spendiness of AAISP etc.
Ordered a bridgeable aDSL modem as I don't need the extra trickery/power consumption of a "super-duper-voodoo-hub". And whilst I was there, ordered some 4 gang extension leads with IEC 14 connectors so that I can run the modem/switch (along with the servers) off the UPS backup outlets and the nuc/brix/pi from UPS surge protected outlets.
Some ASUS iKVM modules for both servers so that I can has out-of-band management also made the list. Mainly because the server boards were EOL ~2 years ago and the modules are getting rarer/more expensive. With the firewall virtualised on one of the servers, I'd still be limited on what I can do away from home unless I switch to a highly available pfSense arrangement.
So that leaves me contemplating some more spendy stuff, which will inevitably have to wait a while. The main server has an 'idle' power consumption of 21W* with the quad port NIC - I'd have to change the PSU, boot disk, CPU, fans in the backup server as well as adding RAM/quad port NIC to get it's power consumption down to something reasonable, whilst making it suitable for running another virtualised firewall. I'd also have to obtain a new switch as I'd need 6 ports just for the firewall->WAN connections. Maybe it's overkill, but the server's gone down unexpectedly twice the last two years and that represents a major inconvenience when I'm on-call.
*Could be less with a modern NIC