If the engine revs increase, but road speed doesn't, then I would say you have a slipping clutch. I really can't see any dealer taking the gearbox out and refitting it to say there is nothing wrong with it. Any serious mechanic would advise replacement of the clutch while the gearbox is pulled out.

Thanks for your reply.

It is a manual Focus.

Once it happened soon after getting into 6th gear, other times it has been in 6th gear for some time.

When it happens there is no sign of it other than you go to accelerate and the rev counter just jumps round. The gear stick has not moved. Whilst slowing down I can move the gear stick to 5th and when I take my foot off the clutch, the rev counter moves up and down freely but there is no engagement. There has never been any problem with movement of the gear stick or getting it into the right position for a gear.

This sounds like a selector fault inside the gearbox. Typically, a selector selects one of 2 gears - so 5th and 6th are probably paired, as are 3rd and 4th etc.

Can you engage 4th as the car slows down, or do you have to wait until the car stops completely to regain transmission ? Perhaps the whole selector mechanism fails until the output shaft stops.

This doesn't sound like a faulty clutch to me - it sounds like you need a new gearbox. A gearbox specialist could repair it, but only if the parts are available from Ford, unlikely these days.