"slipstich it in" - that's what I had in mind, Welshwife, easier to adjust as you go.
It would be more difficult to add it from the inside with machine stitching. But there's already front and neck facing to attach to, and handsew to the armholes.
I'll have a look at that sewing forum eddiecat.
BTW I found the Irish tweed I mentioned earlier - not too expensive:http://www.ebay.co.uk/itm/3m-Irish-Donegal-wool-tweed-fabric-material-for-coats-suits-150cm-wide-label-/281924873162
This is the one I used for my coat.