Like Kurnal I only have a basic understanding of the DSEAR Regs, but I don't think the philisophy ought to be:
"as long as the guy who brings it to site does a risk assessment, then it is ok"
It should be more like
"this is my site and I want to know that there is no alternative means of doing the tasks without the need for these danngerous gasses before he even thinks about bring them to site".
I'm not sure the bloke who brings it to site the best person to assess the risk, as he is unlikely to know what else is going on there.
And to add to Kurnal, not from a legislation point of view, but just a pragmatic one, it might not take a failure of the safety systems associated with the welding to cause a problem, the problem might be a fire starting nearby from another source, then there would be a problem. The rules about quantities of flammables are a good benchmark, but really the starting point should be to try and remove any flammables, the flame proof cabinets are further down the ranking in the list of best control mechanisms.