Thanks for the reply, Chrisee. Having checked my banking app, I can see the last payment Sky took was 26 May, so that does indeed tie in exactly with the SSMTV app stopping working over the weekend.
Very frustrating that Sky have chosen to do this. Clearly their prerogative to do so, but when they set the sports packages as basically the final add-on to their TV subscription, the jump in price for getting the full deal is enormous. Technically the cost itself isn't a barrier for me, if I choose to do it, but it's the value received in return - I've zero interest in watching anything else they have outside of some limited sports broadcasting (getting to watch some cricket, and for my wife watching the F1) , making it a very poor value return.