That's true, John Ballance *did* mention Simtec (so I don't think Castle have a "problem" with Simtec doing the fix).
Castle for their part are offering the service for free, it's also within A6's/Simtec's rights to offer a charged service for the update (as they otherwise are *not* obliged to do *anything*). I suppose whichever you select would be more dictated by Geography.
The biggest cost for people far from either Castle (or A6/Simtec) is transport (the 50 quid don't matter when couriering it both ways will cost more than TWICE that). Which brings me neatly to an unmentioned third alternative.....
I've been using my Iyonix since October 2003, it has crashed only a very few times (I would hardly describe it's behaviour as "unstable" - then maybe I am not doing the sort of things that would provoke crashes). In my case I'd be tempted to just leave it *as is*, and I suspect others may do likewise.