Wish I had not signed up

[#1]

Well something is different.

You should really leave the Plusnet router to do all of the IP address allocation - unless you know exactly what you are doing.  Whatever you do, do not have more that one DHCP service giving out the same IP addresses.

If you MUST do that make the DHCP pools /25 or /26 but all with a sub-mask of /24 and the gateway being 192.168.1.254

I thought I knew exactly what I was doing, but one of the routers was also misbehaving and I ended up with a dog's dinner - some devices connecting to the Plusnet router were getting IP address allocations from the wrong DHCP server - which was also publishing the wrong gateway address.  Sounds very similar to your issue and it took me a good while to fix it (as advised above).

[#2]

It would help to know *exactly* how the network is configured. I thought previously it was stated that no other devices were in use, but now we've established there's a secondary access point.
What is connected to what, and how?
What is the make/model of the access point?
How is it connected to the hub?
I would suggest taking things back to basics. Remove the access point, powercycle the Plusnet Hub, powercycle all of your connected devices, recheck the situation and if nothing looks untoward, then factory reset/reconfigure the secondary access point and reintroduce it to the network.
Do not run DHCP on both router and access point unless you have a very good reason for doing so.

[#3]

Port 1 goes to a net gear switch,did have a 16;way switch,have now changed it to a 8 way switch,one of the outputs goes to a Wi-Fi access point,originally I thought the problem was just on Ethernet,but having the same problem with some devices on Wi-Fi using the plusnet Wi-Fi 
Today been removing the devices that have 192.168.2.***  IP address and change them tom192.168.1.***,but they don’t seem to connect,even though they say they are connected 

[#4]

How have you been 'changing the IP addresses'? Is there any reason you are not just allowing the hub to issue IP's?
My recommendation still stands to strip things back and reconfigure from scratch. Something somewhere is not happy, regardless as to whether or not things did or didn't work with previous equipment.
What is the make/model of the access point?

[#5]

Generally you shoujd do one thing or the other - not both, unless you know exactly what you are doing.

Either allow the DHCP server to allocate addresses or burn them into each device manually.

If you need devices to have a fixed address (for example as network service) then use DHCP address reservation.

[#6]

@Jasperjones 

It is possible to have two networks if required. In this configuration it is important that the wireless network names for the Hub and Wireless-N are not the same.

Using Wireless-N as a Wireless Access Point and not a router would be the best advice.

Using Wireless-N as a router introduces double NAT to the devices attached to it, this could cause problems

I would recommend setting Wireless-N IP Address to be Dynamic so it gets it's IP address from the Hub and set the IP Address allocated by the Hub to be Static.

You could use a static IP Address, you would need to give it a 192.168.1.n* address, mask 255.255.255.0, Gateway address of 192.168.1.254 and DNS 192.168.1.254.

Where n* is an IP Address below the DHCP pool 192.168.1.1 to 192.168.1.63

HTH

[#7]

Will have a go,but seems so strange that the Plusnet router get’s itself stuck with the IP address allocations ,if that’s what it is,when my daughters Vodafone router and my old Talktalk router just worked

Been thinking of buying my own router,any recommendations? The IT contractors at work always recommend Draytek 

[#8]

I agree, to me the way the Hub Two IP Address allocations appears to be random with no sense or logic, the Hub One DHCP Server was much easier to manage.

Depends on your budget on what to buy, some Draytek products are expensive. I've successfully used TP-Link devices, others are available!

If you have space for multiple devices you could buy a router only using the Hub Two as the modem (bridge mode).

You could consider a Mesh system, just make sure they support a PPPoE connection, using the Hub Two as a modem.

A Mesh system or router only device would be future proof for when you go Full Fibre.

HTH