When flight planning using the Ipad I noticed that several disused airfields are not shown, although when checking on the PC flight planning they are shown (e.g Hixon in Staffordshire).

I can see it fine a map scales of 1:500,000 or lower on my iPad (retina). I don't have a mini to hand in order to compare, but I suspect it is being de-cluttered due to conflicting with the label for a nearby airfield.

Still no good when zooming in or out.

That's very strange. I use the ipad mini and all is ok on mine so I can't see why some show and others don't.

Does turning the minimum runway length off under map layers >airfields >minimum runway length >not set - help at all?

Excellent, changing the minimum runway length to not set has cured the problem.
Many thanks