So, I haven't decided yet whether I'm going to press ahead with making more Super Barns or finishing the ones I have part built, but in a few minutes I had free a couple of nights ago I decided to make the modifications needed to the end pieces to make a model of 103.
As you can see from the picture above the windows at either side of the corridor connection are not so tall as on the later carriages - 121 & 108.
Fortunately it proved simple enough to add in a couple of pieces of styrene and slice off the small sections of beading detail at the top of the original window and replace them with pieces of thin strip.
I have also had to hack away some of the double door castings.
On 103 they never bothered to fit a wood-framed droplight in the narrow door so they had to be carefully cut away.