I'm very happy tonight because just over 1/4 turn of the 'screen' trimmer
has solved the problem. It has got the blue to turn off properly in black
parts of the picture, but the blue gun is still extremely strong compared
to the other two. I have been able to get a reasonable colour balance by
changing the engineer settings with the remote.

My conclusion: the tube is not dead yet, but how long it lasts is anyones
guess.

Update: how long did it last? Until mid-august. After making the adjustment
the picture was
OK until it suddenly started getting an overall blue tint again about the
end of July. I tried another adjustment of the 'screen' trimmer but this
only made the picture very dark and contrasty.

Conclusion: I only extended the life of the TV by about 3 months.

How I have fixed the problem: I bought another TV!