To be honest, Pete, I think you'd do better starting from scratch than attempting to convert a shirt into a jacket. For one thing it has no handwarmer pockets, and when I had a look at one of the Westwinds shirts at a shop in Chester (can't recall the name of the place now, sorry) I wasn't terribly impressed with the breast pocket design. No expansion pleats, and they seemed to be sewn rather flat, which either reduces capacity or else means that things you put in the pockets tend to stick into your flesh. If you do opt to convert the shirt then you'd probably want to buy at least a full size larger than your usual size, but I'm really doubtful that a conversion could ever be wholly satisfactory.
I'd advise either looking for second-hand garments or taking an old coat as a pattern and buying some Ventile material, perhaps even offcuts or cosmetic seconds. Even paying someone else to sew it up into a coat would probably cost less than buying new, and you'd have the opportunity to customise the features to suit yourself. If you intend to use the Ventile for a waterproof then go for the minimum number of seams you can manage, without compromising fit and mobility.