Emhar: Mostly British armour including several Type IV and V models of the heavy tank including the male, female and 'tadpole' tail in both 1/35 and 1/72.

RPM: Mostly French armour including several varients of the FT-17 light tank and a couple of Great War trucks in 1/35 and 1/72.

Tauro: Two versions of the German A7V heavy in 1/35th.

I've only built one of the Emhar male Type IVs. Not a bad kit but it's not exactly Tamiya quality either.  

Hey Dave, I've been thinking of joining that GB also!  Not having any WWI armor kits I had to look around and I found the following 1:35 kits:

Emhar - makes several kits of the British WWI tanks: the Mark IV, Mark V, & "Tadpole" medium tanks, as well as the Whippet light tank.

Tauro - makes at least two kits of the German A7V Sturmpanzerwagen

RPM - has released a few kits based on the French Renault FT-17 light tank.

There are probably others in plastic, as well as resin kits.  Don't know the quality of the above kits or how easy they build up.  Hope this helps!

From all I've heard, all those kits are 'Fun' in the wrong way.  None of them are new, or great; I've particularly heard a lot of horror about the A7V from Tauro.  If you want resin, check Commanders Models, they're back and have some cool stuff.

The Tauro A7V is a nightmare. While it comes in two boxings, the kit inside is exactly the same except for decals. The RPM FT-17 tanks are also basically the same kits with just some minor pieces parts exchanged. They issue umpteen variations, so chose your poison.

This is the site you want to visit if you have questions regarding WW1 armor models: http://www.landships.freeservers.com/ 

Click on the "kit lists" link then chose 1/35 scale to get a more comprehensive list than what guys here will shoot off from memory.

Airfix makes a 1/72 Mk.I.

That's one that I'll be building.

Emhar's kits aren't bad, some work required, the vinyl track are horrible though, AM tracks were made, not sure if the are still available

I think Friul has the metal track for the FT-17