Thank you very much for taking the time to help with this, glad to say the problem is solved ;D Your last post prompted me to try something.
I powered off the modem and back on, once it came up I checked and had lost internet except for the ability to ping 8.8.8.8.
I was pinging via IP address, and as I said before the only one that worked was 8.8.8.8 which was my gateway monitor IP address. So I changed the monitor IP for the gateway to 8.8.4.4 and could no longer ping 8.8.8.8 but could now ping 8.8.4.4.
Whilst I was in Edit Gateway changing the monitor IP address I noticed a check box to make it the default gateway, ticking that fixed the issue once applied.
Don't know if the behaviour I was seeing is expected behaviour or just some weirdness going on, either way I'm very pleased its sorted.
Thanks to everyone that tried to help.