Thanks for your replies.
The layout had been shut down for at least 18 hours. The HC-2 the same.
The engine I was adding was number 12. The HC-2 let me add it to the roster, but if I tried any thing else, I got the message 'too many engines are active...'
If it happens again, I'll try bringing up all the engines and then delete them from the running roster by making sure the throttle is at 000, and then close the page. Then wait 10 seconds to do the next engine.