sleeper.scot is the site Will only offer sleepers to passholders and the recline seats not.  Sleepers are some 120GBP!!

BUT it has-this is the island where everything has to happen in another way, uses anOTHER rule-the old rule in fact. You have to use the NEXT=arrival day written, thus your prob is solved! And you can plan a very long onward trip next morning.

Sorry I dont understand how I can book the train for the next day?

The answer is that there is a unique rule for Britain that the night trains uses the day of the arrival of the train as the travel day. In the rest of Europe it is the departure day that is the travel day for a night train. In your case the the travel day for your night train will be the day of the arrival in London, which is what you want.

Excellent...thanks...now if I could just get the app to accept my login!

Which login? Do you have a screenshot?

when I use laptop to access Interrail site- have changed password and it wont then accept it

Ah right that's not the app… can't help you there I'm afraid.

Instructions to book this sleeper can be found here. Do check the price of a normal ticket because that can be cheaper than a pass reservation.

The timetable in the app is incorrect in suggesting you have to change in Carstairs. That is a direct train of course.