Well it worked (eventually)
here are my tips.
1) Make sure you get the background colour properly into the computer, scanner/printer differences mean the shades differ slightly.
2) Use photo paper for tests as colours are different to plain paper, transfer paper is very similar to photo paper.
3) Use 720DPI or higher when scanning a painted item, also scan some existing transfers to get the correct size panels. You won't be able to use them but are great for getting the correct size.
4) Copy the BMP if you cut and paste you lose the resolution
5) Slice the paper in half and use A5
5) Don't use black use a dark grey - less likelu to run into the white
6) Leave plenty of white space.
7) Leave at least 2mm border of wagon colour beyond the new transfer
8) Leave to dry for a few hours.
9) Use a spray can of acrylic varnish designed for the job, acrylic floor polish causes ink runs and cellulose laquer cracks and allows water to run in.
10) Do not soak for long and only soak from behind.
11) Use weathering to disguise any colour differences.
12) When producing them make sure you have variations, use pictures, also while you are doing it you might as well do transfers for more wagons and store them.
13) Arial is the nearest font to BR found on a Windows PC, with 720 dpi I use 20 point, 28 point and 36 point.
14) Do the lettering as white on black on a mono BMP to avoid the edge blurring normally found in mspaint. Cut and Paste into your BMP
15) Use BMPs as JPGs will change the colours due to compression.
Engineers Olive is well to dark when scanned and printed, I suggest creating a large amounts of tests on a sheet of photo paper.