I think the saying is… it never rains…
Thank you for your suggestions. I have taken a couple of photos that will answer some of the questions but, needless to say, my camera picks today to pack up!
I'll try again tomorrow to put the pictures on here but your answers got me thinking.
The tubeplate has one end of the offending tube in it, joined to a boss tapped 3/8 BSP. A 1/2" hand reamer just entered and went in as far as the start of the tube. I then used a worn, but newly sharpened on the end only,1/2" end mill, used in a tap wrench, and cut away the tube so that I can see a gap all round it.
I shall then do as Jason suggested and heat up the other end of the tube ( which I had fortunately left oversize) and pull out the tube. Thats the theory anyway. I'll let you know how I got on and hopefully get the pictures on.
Brian