OK, few things left to try
a) giving both the printer and PC static IP addresses, and possibly changing the DNS server address to that of your router.
b) installing SAMBA and configuring your Windows domain (and possibly netbbios name through there)
c) seeing if this will configure through CUPS
Do you know how to give both the PC and printer static IP addresses ? (preferably ones that won’t conflict with your routers DHCP pool)
I seem to remember that the printer address is static - from your advice when I first installed it so long ago. I think the router addresses start at 200 which why I chose 201 for the printer.
I can change the PC from auto DHCP to manual in the Networks menu, but I’m not sure what to enter.
OK, I’m going to assume your routers IP is 192.168.2.1 if not change both the instances of that below to whatever it is … if it’s not even in the 192.168.2.x range, STOP now and tell me what it is.
I’m also assuming you don’t have another PC with a static I of 192.168.2.225 … and that is outside the routers DHCP pool
I thought you said your printer was 192.168.2.210 not 201 ?
On the IPv6 tab set
Method: Ignored
on the IPv4 tab set
Method: Manual
Address: 192.168.2.225
Netmask: 255.255.255.0
Gateway: 192.168.2.1
DNS Servers: 192.168.2.1
The server address is 192.168.1.254
The server address pool is 192.168.1.64-253
The printer address is 192.168.1.201 (as in previous posts, but I had put .2. rather than .1.)
However: I am getting seriously out of my depth here and feel very reluctant to start assigning static addresses all my kit just to solve a problem for this one, very old, PC that will be replaced soon. It really is very easy to screw up a router!
Understanding networks is on my list of things to do, but I’m a slow learner, so I hope you don’t mind if I chicken out at this point as I really don’t understand what I am doing, and I’m not comfortable with that. I shall upgrade the computer instead.
My thanks as always for your patience.
Keith
Okey dokey 
If I’m understanding you correctly, the Printers static IP is actually inside your routers DHCP pool … and it shouldn’t be
it should be something like 192.168.1.50
And you’d set the PC as
IPv6 tab
Method: Ignore
IPv4 tab
Method: Manual
Address: 192.168.1.51
Netmask: 255.255.255.0
Gateway: 192.168.1.254
DNS Servers: 192.168.1.254
But up to you 
…and that worked perfectly!
The problem all along, as you have just pointed out, was that the printer address was within the DHCP address pool. I am amazed that all other PCs have been able to access the printer without trouble.
I notice that my P3 laptop has also set itself to the same manual status and the same settings entered on the P5 PC. I really have no idea what’s happening in this dark art but I am once more greatly in your debt.
Thank you, Mark.
Great … happy to hear it’s all now working.
So I take it you can print now ?
Yes, I can print with impunity. And they can’t touch you for it.
I am at this moment searching the web for network tutorials in a bid to understand what I’ve done.
OK then … see you in a year or two … then you can teach it me :o
Ho! Ho! Ho! I’ll be back all right, but asking more questions, as usual. Speaking of which … but that’s another Topic.