I've finally fitted Kadees to my old Bachmann class 24 and Vitrains 37. Many thanks to everyone who has helped with advice on how to achieve this.
For the Vitrains 37, I pulled out the tension lock from the unusual mounting. A Kadee number 20 had its 'legs' cut off, being careful to make the cut at 90 degrees. A file was then used to make a small fishtail in the Kadee, similar to that on the tension lock previously removed. It took me three attempts to get two couplers that fitted. Fit the modified Kadee into the coupler mounting, checking with a height gauge that it is at the right height, superglue in position. Once sure it was OK, I strengthened the joint by adding some more superglue and sprinkled on some 'Rocket Powder' which makes a sort of superglue filler. PS I've lost two of the class 37 buffer heads, any idea where I can get replacements?
For the class 24, I removed the long coupling plate by undoing the crosshead screw. I then pulled off the tension lock hook and cut off the tension lock loop. A Kadee number 19 coupling was simply superglued on top of the coupling plate. By pure luck, the coupling is the right height without any adjustment needed. I did remove a mm or two from the underside of the buffer beam to allow for gradient changes. Again I strengthened the join with 'Rocket Powder'. When dry I refitted the coupling plate, That's my two 'difficult loco' coupling conversions done.