After some thought, I purchased a 400mm ER32 collet chuck and a suitable Myford pattern back plate, from Arc Trade Euro. I have just finished machining and fitting it. On testing the internal taper at several points, I found the maximum runout to be about half of one tenth of a thou! Very pleased with the result. I have no connection with Arc Trade Euro, other than as an occasional and very satisfied customer.
In order to fasten the chuck to the back plate, I required three screws. I went to Apex Fasteners on Slough trading estate in Berks. The chap that served me found the correct screws for me, didn't complain when I changed my mind about the length, and when I came to pay for them he said "put fifty pence in the Poppy Appeal Box". I gladly put more than that in. What a great firm! I have used them for several years now and they are so helpful, knowledgable, and have no minimum charge. I know that few readers of this forum will be able to use them, but I just wanted to say how great they are! Such a refreshing change. Again, I have no conection with them.
Maurice