If two crews out of four are declared 24 hours every day how do you get leave weekends etc
That's not the case and never has been - 2nd crew only available 8am till dusk (though never finishing before 6pm in winter and never later than 10pm in summer). That said, when large incidents have cropped up at night, we have never failed to find a crew for the 2nd aircraft even though there was no crew on standby.
when and if you really go down to four crews you will not be carrying out the standby role in the way you do today.
Really? Certainly the intention of the powers that be is to preserve existing levels of standby. It will certainly make it harder to book leave and courses with a smaller pool of people, but 1st and 2nd Standby is likely to continue unchanged except in exceptional circumstances - as has always been the case.