Roger we can accept FL390
ABC123 maintain FL350 expect further higher in 5 minutes
Maintain FL350 ABC123
NATS 'Best Practice' in the UK is not to include the actual Flight Level if using the phrase 'Maintain'. This is because there is a chance the wrong level might be mentioned (human error is always a possibility) and in some States the pilot can take the 'Maintain' to be a clearance to change level.
Plug the Swiss Cheese holes
