Screwfix foam gun problems!

Has anyone here tried the screwfix foam gun, you can see it here http://tinyurl.com/o70i
I've gone through three cans of foam and two guns, one of which was supplied free by screwfix but I can not get any flow from the damn thing. I just wondered before I get back to screwfix if anyone else was having the same problem?
Also I noticed that the gun in the picture on the website is an all metal affair whereas the supplied gun and the one in the catalogue is mostly plastic. Has anyone received the metal type?
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Well I didn't put any through it from new if that's what you mean. I did put some through it after it didn't work the first time and it sprayed through quite nicely. I eventually managed to get a real wimpy flow by holding the trigger in while screwing the foam can on, but it was totally unusable. I really think there's a design flaw with the gun. I'll have to tell screwfix!
Steve
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