Landships II

Iron Duke would be one of the easiest builds for the Airfix kit. A quick list of corrections (from memory) would be:
1) Leave the tailwheels off.
2) Leave the silencer off and replace with inverted "V" exhaust covers.
3) Scratch-build a rear stores box.
4) Drill out sighting slits for the 6-pounders.
5) Behind the roof hatch there are two rivetted strips running across the roof and two running along the roof. Remove the strip running along the roof closest to the hatch. This strip appears on various drawings but never on a real tank.
6) Now it gets tricky. Depending on how much work you want, the rivet pattern on the body is more or less accurate for Mother. You have at least three options:
                   1) Leave the rivets alone and hope no one notices. Easiest method.
                   2) Remove every other rivet down the body side. Looks closer to correct than leaving them alone.
                   3) Sand off all the rivets on the side of the body and re-rivet the whole thing. Definitely the hardest method!
7) Rivets again. Around the track horns there are actually half as many rivets as there should be. The two ways of dealing with this are 1) Ignore it or 2) Put a rivet between each of the existing rivets.
8) Rivets part III. Above the vision slits for the cab front there are eleven rivets. On a Mk II (and III) the end two rivets are closer to the rest of the rivets. Again, ignore problem or move rivets.
9) Rivets part IV. The roof hatch has none and needs some. Also the top of the cab roof needs eleven in the same pattern as the front of the cab.
10) The towing point could use a little work. Either scratch-built PE or styrene will fix it and a bent brass rod will serve for the towing bar.
11) Riveted strips are needed on top of the sponsons and between the 2nd and 3rd exhaust port. L section strip for the sponsons and T section between the exhaust ports. Or you could leave it off and hope no one notices.
12) Rivets part V. While you're messing about with the sponson roof, you may as well rivet the top outside edge.
13) Cab roof hatch: Iron Duke didn't have it so you'll have to cover it. The only Mk I or II tanks that did have thet type of hatch were the supply tanks.
14) On the rear plate you'll need to 1) Fill the holes for the hydraulic ram and 2) Make a small square plate to go near the top between the radiator exhaust vents and the rear door.
15) While you're at the back of the tank, you'd better fill in the holes for the tailwheel mounts on the inner rear horns.
16) If you're really keen, the underside of the hull needs panel lines scribing and more rivets adding.
17) The cab sides need vision slits and pistol ports.
18) According to Mike Cooper, the roof hatch is located too far away from the centre line of the tank. He's right of course, but it's a lot of work to relocate. It's your call if you want to move it.

That's all I can remember off the top of my head. I think the sponson needs work re: pistol port arrangement port and starboard but I'd have to check.

I hope this list isn't too daunting. A lot of it is small stuff that nobody will notice anyway. 

The radiator drain aperture on the lower rear hull also appears to have a small square protective plate (along with the radiator filling aperture above it, as Mark has mentioned). Thanks, Phil, for posting that great clip of Iron Duke.