Thankyou everyone, I do appreciate your time and efforts and, once again, I am learning a lot.
No one here would go down the Blue butane bottle route and all would have gone red bottle.
Finally photographing the sticker on the locker door, shows that the original were 3.9 kilo bottles, which looking online are red propane.
I am fairly logical when I try to identify issues and just made another trial.
Having been out in the van today and been very warm, I thought I would try the 4.5k cylinder again to see if there is any difference with it being much warmer.
Absolutely no difference.
The video attached shows gas hob on and water heater.
As the water heater kicks in the gas hob flame drops and the fault code shows on the panel.
Just looked and videos don't appear to be allowed so, no video.
Anyway, as mentioned, I try to learn and although I will be changing to a red cylinder in some way shape or form, does any one know why, when it's nice and warm that the blue bottle seems severely lacking in pressure or something else.
The route I think I will try to take is visit my local gas stockist and ask if I can try some sizes in the van locker.
I would like to get as big as I can manage to manipulate without doing myself damage when it comes time to change. With such tight space, this was the reason for the small bottle.
Lastly, as mentioned above, changing from blue to red and needing a different connection.
Is there a converter I could just plug / screw into the end of the hose to be able to adapt the hose to fit red or is there a kit/hose I can buy and fit.
I really have such a low opinion of any motorhome / habcheck / fitter / installer that I really can't do a worse job. (famous last words)
(With everyone's help on a previous call for help, the crash data solution was fantastic)