iMessage double message or SMS green - Tesco Mobile - iPhone

HI All

So this is the scenario:

1 x iPhone SE

1 x iPhone 7plus
Both ios12.01

Both Tesco Mobile Contract Network. (Been on Tesco since around 2014)

1 x iPhone 7plus on 2-3 bars 3G or 4G network:

1 x iPhone SE on similar network or Home Wifi Apple airport.

When using iMessage all can seem fine then out of the blue the mobile pauses when sending and then sends the message as a SMS so that the user the other end get 2 x Messages. 1 x iMessage and 1 x Text SMS. Exactly the same message:

I have:
1. Run remote diagnostics with Apple support, both handsets are fine no Hardware issues
2. Reset and restarted iMessage activation with 2 x Email addresses and main address is mobile number
3. Contacted Tesco Mobile and they have looked at Network, all fine. (nearby mast fine)
4. Had 2 x New SIM Cards
5. Changed APN settings to setting on Tesco guides website, these are slightly different to default settings on device.

Just not sure where to go next. If I turn off iMessage the SMS green texts work fine all day long. If I turn iMessage back on they can work fine then suddenly fail for no reason. Halfway through a chat the message can go green my end then appear as a double message on my wife iPhone SE.

I think its the network, I think that data is denied or paused or not prioritised on the iMessage service. This makes the iPhone think that iMessage is unavailable and it sends the message as an SMS instead and yet it still sends the iMessage. Hence the double message at the receiver.

Any ideas? Driving me bonkers!