That depends on how much current you intend to be using. Give an idea of what sort of welding you'll be doing and someone will be able to tell you whether or not you'll get too much voltage drop.
From memory I think they normally say 1.5mm square conductor size for up to 25m with 4 percent drop. 4mm square for up to 50m with 3 percent drop. based on 3Kw load.
Heavy duty somtimes just means the insulation material and not the weight of copper so be careful.
I regularly run a 140 amp inverter off one of these 50m leads, never had a problem and no noticeable drop in performance, just make sure its completely unwound before using it.
Run my Clarke 160TM off a 40m extension all the time at the moment, had it cranked upto full the other day for welding 1/4" plate for some kit car diff mounts.
No problem, no blown fuse, no appreciable loss of power, stuck the 1/4" plate together pretty damned well.
Mines a 40m 1.5mm2 artic rated real from masterplug, cost about £35 from screwfix and has been great. Has an RCD plug as well.
Sorry I forgot to mention that it is only for welding work on a car. So by the sounds of it I should be okay with using one and be able to use normal settings?
I use mine with a good quality arctic cable extension, they are very flexible and I don't have any problems at any setting. As mentioned above just make sure you reel it all out.
I made my own up with 2.5mm² arctic (blue = 240V) cable from local electrical wholesalers, use 16A plug / socket as they are more reliable with constant high current compared with 13A plugtops.