xt brake done a gentle 800 miles, pads are half worn down if that, works fine then after a few uses even on gentle downhills goes very hard and wont pull in towards the bars very much, but still works fine, hose is not damaged or kinked
any ideas please?

sticky pistons? mine seem to do this every 6 months or so. Use a cotton bud with a bit of mineral brake fluid on to clean them up and cycle them in and out a few times. seems to do the trick.

wierd, sounds like fluid getting hot, but then would expect it to go spongy.
maybe try some fresh fluid

Could be pistons. Clean them up and work them in and out a bit. If you've got a yellow bleeding block then stick that in and try the "zip tying the lever back to the bar" trick over night. If any bubbles have got in they'll work their way to the top. If you tap the lever lightly a few times after that, they should be out into the reservoir.

I've experienced similar intermittently on new style levers, never on older levers . My last set did it for a while, then it stopped after a bleed. got a new set and noticed exactly the same the other day, but after a couple of days sitting they are back to normal. I'm going to rebleed and see if that solves it.

BTW getting all the air out can be a PITA: I've got a Shimano bleed kit which though expesnive for a few little bits, makes it so much easier.

Dave