Chain loses grip under load in higher gears When pedalling  in higher gears, the crank or cassette seem to completely lose grip for a second, then grip then lose it again. I could probably pedal all day in 1st and any front ring but when start to go in 6/7/8th it will do it.

This is not a gear change problem, all gears change perfectly and when the bike is on the stand everything works perfectly.

In order to try to cure the problem, the front and rear derailleurs have been renewed including cables, the chain renewed, the derailleur hanger also, but the problem remains.  I tried swapping the back wheel with known good one off another bike to eliminate the cassette, but the problem stayed the same. 

The front crank has a tiny bit of play but the teeth look ok - none obviously more worn or broken than others - the biggest ring on the crank has been bent and straightened, not perfect but it has been working fine previously like that, plus the problem exists on the two smaller rings too.

Any help appreciated
The comments above are probably correct; if your chain is new, then the issue is likely a worn cassette. Because they wear out as a pair, chains and cassettes are often replaced simultaneously. A new chain on an old cassette can lead to skipping, just like an old chain on a new cassette can lead to skipping. 

Another possible cause of "losing grip" could be a damaged or missing pawl inside the freehub body's ratcheting mechanism. If, for example, one out of your three pawls is chipped or has broken off, the freehub will engage fine under light load, but heavier force (like pedaling hard in higher gears) can cause the hub to "freewheel" briefly in the wrong (pedaling) direction.