I was out riding the old beast on Sunday, no issues to start with, lots of slow speed figure of eights etc, then I went on a long trail across a field / meadow .....

When I arrived at the gate at the far end around 10mins later (which was locked) having not used my brakes at all, I had lost my front brake, lever to the bar nothing ......(the brake is usually one finger operation and really good). I rode back and kept pumping it and nothing for a while then all of a sudden it came back and has been fine since ?!? 

 

Air in the system perhaps ? Low fluid ? Doesn't make any sense to me. 

Pad knock-off?

It sounds like something pushed the pistons back into the caliper. A bent rotor will do that as will badly worn wheel bearings or even picking up a small rock or piece of wood that gets jammed into the brake pads.

to make sure there are no leaks while parked grab a handfull of front brake and hold it to see if it goes soft after a bit. If it does then you have a leak somewhere in the system. Could be a seal in the caliper or master cylinder. If the lever stays firm I'd check the front rotor for true and check for play in the wheel bearings. Also pull the caliper off the rotor and look for anything on the pads that doesn't belong there.

Obviously check your fluid level and make sure caliper bolts are  tight and the lever has enough free play to open the reservoir port in the master cylinder.