That's why I was surprised by what Dave Yates said, as it looks like a standard repair to me. I assume that bronze welding means the same as fillet-brazing. All steel repairs are done with fillet-brazing because it uses a much lower temperature than welding. You're still going to have the existing top tube, seat tube and bb shell and they have already been weakened once by the original TIG welds. That's fine, but they shouldn't be weakened a second time by another TIG weld, hence the use of a lower-temperature method for repairs.
Since Thorn/SJS are in Bristol, the natural thing for them to do if they no longer have frame-building facilities of their own is to send it to Argos Cycles for the repair and respray. You ought to insist on 853 tubes being used, as Argos would use 631 unless Thorn paid the extra.