David,
I also have the 'Preserved' book, I must have a good look at it again and try to actually preserve something before the end of the year
Your talk of smoker brought back many memories of my childhood in the North West Highlands of Scotland! My grandfather constructed a simple smoker using an aluminium wardrobe from an old caravan and some oak chips, he used to smoke enough mackrel for the entire village! He drilled holes in the side of the wardrobe, threaded the fish along a wire (coathanger I think) and then hung it inside the thing.
I've just spoken to my dad to get his words of wisdom on the matter, I'll tell you what I can remember (in no particular order!)
for cold smoking, you need to soak the mackerel (or other oily fish) in brine for about 20 minutes before hand, if you're posh you can add in juniper berries and the like. The fish then need to be put out to dry until they are all tacky, then they will take up the smoke really well.
If you are cold smoking it can be tricky to keep the fire smoking at the right temp, too hot and the fish will cook far too quickly and end up on the floor and not smoked.
My grandad used a large metal pipe to relay the smoke into the bottom of his wardrobe type thing, that way it was cold smoke and the fire was seperate from the fish.
My father said he would give me more details on the smoker and even draw a picture for you if you are interested, I guess it's similar to the one in the preserved book though