Well, the mesh borked so terribly that I figured I would probably spend about as much time fixing it as I would starting over, so I started over.
Well I was wrong.
The last version of the horns took me about 8 months all up. I don't want to know how long I would have taken to eventually fix the mesh as I know I spent a good week or so experimenting. Starting over this version took me...
...wait for it...
...3 days (or the equivalent thereof as long blocks of work are hard to come by).
I sized Red using my usual trick (a box at the correct height and then resizing the base model), removed the wing membranes (the cause of the mesh tears I didn't see last time) and then just threw the lot into Sculptris (no problems this time!) with the intention of doing ALL of the detailing there. It took me a few goes to start the horns from sculpt but after a couple of hours I got the hang of it.
Spent probably most of a day getting the shape right and simultaneously raging and being ecstatic about how much easier it was doing it this way as for starters the shape was closer to what I was trying and failing to get (though I suspect the shape I actually wanted is not actually possible) with the spline guides and poly modelling. Perhaps I can sculpt after all.
And mostly done (I say because I will probably keep touching it as I go through and do the rest of him (again). I need texture on the crest and at the tips but not quite sure what I'm going to do about it yet, whether it's going to be bump map or if I scribble something in. Currently at 177 686 tris.