Never really had an issue before, normal descending speed would be in the 70kmh mark but yesterday was descending at just over 80kmh, unfamiliar corner came up and had to brake a little to ensure I was going to get around it, this seemed to setup a brown short moment of front wheel shimmying and required me to stop braking and go wide and onto the otherside of the road while feathering the brakes.
WHY did I get the shimmy too much weight on the front caused by not shoving myself back on the saddle? too low on the bars and not enough control? really would not like to repeat that moment again.
Really lucky that I had no traffic coming on the other side, dont mind the crashing into barriers and hitting the ditch, part of the calculated risk you take when racing and trying to make time on the descent, but into a car is NO FUN AT ALL.

P.S telling me to go slower is not an answer that I am racing and just need to know how to minimise the risk of the shimmy.

The thing with speed wobbles, you never really know when it will happen.
The only thing to keep in mind is that changing your center of mass and keeping a LOOSE grip are the important parts. I know it's counter intuitive that keeping a hard grip only makes it worse, but that's a bike for ya'. And yes, it's easier said then done.

Have you thought about another pastime? Maybe chess? Origami? Line dancing?

That can happen if your headset is just a very little bit loose. Using the front brake can cause the wobble when there is the smallest amount of play in the headset.

Headset is good and tight, it was definitely a left to right motion as if the wheel was going to wash out on me...as to another pastime nah just love racing too much nothing else gets the heart rate up there...AEO, loose grip was not going on at that point! it was a oh ***** moment and I obviously tensed up and made things a lot worse,
In hindsight I reckon that if I just leant her over a lot more I would have made it with no braking but at that point I did not know how much tighter the corner was going to get and panicked :-( which is not like me but as it was at end of ride I was perhaps a little tired...or just getting old and no longer have the guts to do the high speed corners

There are quite a few threads on this subject. It's impossible to know what caused your shimmy, but the main things to keep in mind is that you need to be relaxed, your weight needs to be back, and your knees should be in on the top tube. If you tense up and let your bike go into a harmonic vibration (which is what a shimmy essentially is), your bike will go into what I like to call the death dance. It's really no fun.

I had seen a few threads alright on the high speed wobble,just was not sure whether a shimmy under braking was different to just a resonance for no reason if you know what I mean

Braking can certainly aggravate problems, especially if you have the slightest issues in the braking surfaces (particularly on the front).