Cramp ! Is it my body telling me to ease off a bit !

I tend to crap only in summer when I'm sweating more. There are companies that can analyse your sweat and help you that way, or go to a doctor as suggested above. I used to use electrolyte drinks but found they gave me stomach ache on longer rides so started using just water, that's when the cramps started being an issue.
A friend put me onto saltsticks, other brands are available. Not only do they prevent the cramps but they seem to make the recovery a bit easier too. Main bonus is that they don't make my tummy hurt too!

Guranteed cure: Stop at the first pub you come to. Have a pint of bitter and a packet of salt and vinegar crisps. Repeat at every pub you pass until you get home. Works best if you plan your route to pass multiple pubs.

Im with Madcarew on this. Cramp is primarily based on over-use, or over-stress as I call it. 

I used to get cramp a lot on the bike, however over the years of steadily increasing fitness, and also more and more refined bike fitting, this problem has all but gone.

Of the above, I'd say bike fitting has had the biggest effect. Reduce the overall load placed on a given muscle will reduce its susceptibility for cramping.

However, the knock on of this refinement is that when I reach the limit, the cramp can come, and when it comes it has the potential to hit a lot of muscle groups simultaneously. 

Thats not to say other factors won't contribute... as touched on, electrolyte imbalances can influence suspectability, however make no mistake, a muscle, or a group of muscles fitness for a given purpose is the largest contributing factor. 

I get night cramps without any obvious trigger. Back in the days of sheets and blankets I always used to untuck the bottom of the bed so I could stick my feet out and stretch when it inevitably hit. I think I'm just predisposed to it, I also get crawling twitches in my calves which aren't painful but look like they should be - it's called myokymia https://en.wikipedia.org/wiki/Myokymia

Sometimes the cramps are in response to a long day on my feet outside where I've probably got a little dehydrated but equally it can happen after a day sat around doing nothing. I just put up with it and know I can usually stretch it out and just feel a little sore next day. Thigh muscle cramps are the worst.

Because I'm aware of the risk I tend to stretch on the bike when I get the chance on descents - just drop my heel with the pedal at the bottom of the stroke for 30 seconds or so. No idea if it helps. I also favour spinning a lower gear wherever there's a choice but that's more take pressure of my knees.

You might want to get off the bike and do some stetching and such before you get to the point where you usually get cramps. Not too hard, the point is to reduce tension.
It might work or make it worse.... worth a try, though.

"the idea that muscle cramps are caused by dehydration and electrolyte depletion has a long history but very little evidence behind it."

https://www.runnersworld.com/health-injuries/a20802164/a-cure-for-muscle...

Also by Alex - http://sweatscience.com/cramping-in-ironman-triathlons-not-dehydration-o...

Yes. It's your body saying "This is hard work". All the research says that cramp is entirely neurological. There are no drinks or foods you can eat that will correct an electroyte balance or anything to make a difference. I believe tramadol can help.

You can think of cramp as a kind of overuse syndrome. At a certain point of fatigue (or other neurological stress) your muscles start sending back a message "I can't do this at this level any more". and so you generally have to ease off.

You can improve the situation (indeed, possibly even fix it) with training. As you say, on steeper hills you get cramp, when your muscles are working really hard. Doing steep hills multiple times in shorter training rides will help, as will sprinting multiple times in a  shorter ride. Doing back to back rides of 60 - 80 k may also help. Really it's your body saying "This is my limit at the minute" so you do normal training that will extend that limit. Also, because it's neurological it can literally be corrected by your brain to a large degree, so being relaxed and not getting concerned about it will help!!

"I have drinks with the electrolytes and eat enough stuff."

I had the same issue years ago where I retained potassium but released more sodium the further I rode. So, you may be depleting one or more electrolytes - sodium, potassium, chloride and co2 - or may be imbalanced from medications/supplements you take. A doctor can order an electolyte panel to see where you're at. You can then determine if additional electrolye(s) need to be added to your ride drinks/food beforehand.

Old?

https://www.runnersworld.com/advanced/a20783751/keep-from-cramping/

Old cyclists' folklore says eating bananas before, and during, exercise fights off the dreaded cramp. Just might be worth a shot.

Have you got the correct gearing?

If you aren't spinning the gears enough you'll cramp much easier.

Calves or thighs? I find I'm the same - about 50 miles is my limit. Mine tends to come on when I stop at junctions, particularly calf muscles when I stretch to dab down (I tend not to drop off the saddle). Less likely to get it on the ride itself. I've always suffered with it, I get night cramps in calves (at the back and also down the outside of the calf muscle). It can also be triggered by posture - e.g the hard edge of a chair can set it off in my thighs. I drink plenty of water on long rides (1 litre an hour) but I don't see any correlation between heat and onset. For me I just need to stretch it out and it tends to be a little sore next day, but usually a ride sorts it out.

I would say if you're eating and drinking it's perhaps down to posture on the bike, or your cadence. Higher cadence/lower gear might help.