Thanks for replying Ashley.
Would that the solution were as simple (not that legging up a ladder to tweak a dish is simple). Retuning was the first thing I tried.
However, the Propellor channel is also scrambled on a friends equipment in a different district, from the same date.
I emailed propellortv.co.uk, who responded thus "We have recently changed our transponder details and should still be available on Freesat channel 5147. " I'm not sure they fully understand the details - Propellor was not available on Freesat, only the "non-Freesat" channels on our Humax receiver, and is still listed as channel 5304, but is "scrambled or unavailable". Channel 5147 is an Asian channel Imagine Dil Se.
I could try a manual tune on a particular transponder, so I have asked Propellor tv for their updated transponder details, but I am still waiting to hear from them.
I know I could watch on TV Anytime, but this would gobble up our 2Gb monthly allowance.
Watch this space.