Sealey used to have the manuals on-line, but they moved or deleted them last year. Probably your best bet would be to contact them via their website and ask them to email one to you.
If you can swap polarity of the torch and earth lead (via connections on the front or in the wire compartment) then it can be swapped to gasless.
Malcolm, they haven't moved or deleted them, but you do now have to register on their website to be able to access them. Their website says:
"NEW - Personal Product Services (Inc Instructions & Parts Information)
Register online by clicking in the Register box under Product & Full Text Search. Submit your details to allow you access to our Personal Product Services. These services allow you to gain access to product-linked Instruction and Parts PDF documents and to register your purchases with us online for warranty purposes, maintaining a visible record of your Sealey tools for your reference."