Worn Thumbs, Yetloh, Rafezetter, Eric - thanks all.
I'll try the garage up the road to see if they can braze it for me - they owe me a favour - paying for the job, brazed or welded, would of course be uneconomic.
I did go there yesterday but they were shut - Summer hols I suppose.
The biggest problem that I can see is keeping the two halves perfectly aligned so that the bars can still slide afterwards - I can only see this happening if the cracked casting is mounted on the bars at the time of brazing, and this itself carries all kinds of risks.
Raf - Bristol is too far away unfortunately - 170 miles, a £6.70 bridge toll, and a couple of pints at least, which adds up to a bit too much to be worthwhile.
Well, a lesson learned anyway - when you're buying an old vise, check for cracks!
Martin.