Thanks for all the great advice. We put in a box filter.
Unfortunately Lucifer passed away after a few days

The water params were fine so we could not work out what went wrong

My daughter was most upset.
I decided to try and cycle it further with a tiny feeder (tetra feeder) before getting her another betta and he died too!
I worked out the problem after that...the glass wishing stones!!!!!!!!!!!!!!!!!
The substrate was natural black/brown river pebbles mixed with wishing stones to add a nice sparkle and close inspection of them proved the bottom of them was painted/glazed

It seems the glaze is giving off toxins.
We removed them and re-set the whole thing up again with another feeder and so far this one seems very perky and happy. If my theory on the stones proves correct and he keeps going fine then she can get another betta.
Just goes to show you have to be so careful with what stones you put in a tank, a hard lesson learnt by my daughter and myself.