Slip and nasty fall

This isn't one of those "aren't GatorSkins terrible" threads is it? Isn't the complaint that they acquired a smooth glossy sheen? 

I took a similar-ish sounding spill a few years ago. It's a dip the road with a bend at the bottom that I normally tackled the other  direction.The bike just seemed to get away from me and accelerate, I braked, but I couldn't make the bend and hit a wire fence with posts at a shallow angle - wrote off my front wheel and never trusted the frame again (CF).  A few bruises,  not a show stopper and hopefully not for you either.

As I understand it, a dry spell sees a lot of car gunk builds up on the surface and a bit of rain "activates" it and makes things slippery, though not so bad as to give the effect you describe.  The other time this happened to me is on newly painted double yellow lines, which were damp - I rolled across them and was promptly dumped on the ground with the bike seeming to just vanish to the side. 

+1 for diesel; is the corner on a bus route? They're the worst culprits though there are plenty of others. Often impossible to see, one of my regular routes takes me past a bus garage and however carefully you scan the tarmac there's always the risk of missing the "liquid black ice", particularly if there's been some rain because the water seems to take away its characteristic rainbow sheen. Not a lot you can do about it except take a line round corners where the filler cap wouldn't be to give yourself the best chance of missing any trail from a leaking vehicle. Wishing you a speedy recovery and hope you're back on the bike soon.

In January I came off on a low speed bend just like that - no warning; front wheel just went. Long story short I'm waiting for physios to clear me to ride on my newly replaced hip.

Probably diesel as it was the entrance to a railway station heavily used by buses and taxis. Unfortunately it's just $#!+ That Happens.

I'm also concerned about a repeat so I'm replacing the road bike with a gravel in the hope that the grip will be better.

Sorry to hear this, it could be due to a diesel spill. I sometime see vehicles that have been filled to the brim and then it as it turns a corner in the opposite direction to the side the filler is on, fuel leaks from the filler neck, and if it's diesel, doesn't evaporate. 

I was on a group ride once when about half a dozen of us came off simultaneously. It was May, the road was under trees, we think a huge amount of aphids on the new leaves had dripped slimy tree sap onto the road surface.
Got a dislocated shoulder out of that one.

Sorry to read that, hope you're back out again soon!  Sounds like you're an experienced rider and it was one of those things.

Salt ironically is certainly slippery, or maybe there was some diesel on the approach. 

After a dry period, there is a build-up of rubber and other debris which can be slippery when wet- this can be particularly bad in a summer shower on the Continent.

First up - wishing you a speedy recovery.

The same has happened to me. I don't know whether it is due to the physics of tyres being cold, old, or soemthing like that. My biggest suspicion is that, at this time of year, the roads have a LOT of runoff on them. Lots of bits of tiny gravel, road salt, etc, particularly if there has recently been heavy rain, like mini ball bearings. Just take it easy in the corners. 

I suspect the answer is to build the confidence back up - brake early for corners, relax into them, don't brake or accelarate in corners, get your centre of gravity lower and go on the drops. Relax, relax, relax, and avoid the "death grip" on the handlebars.

Of course, this is easier said than done.