As the previuos message says, thier terms and conditions have to remain the same. You can make alterations through consultation with the cleaner, but the cleaner has to agree.
It is not unreasonable to expect the cleaner to clean windows, providing you are not expecting him/her to to carry out the task outside of his/her normal hours or to climb a ladder or steps (as this would be considered a more specialist task, the cleaning of glass can and often does come under the duties of a cleaning operative).
You can change the specifics of the duties, the title of cleaner covers a wide variety of duties and as long as the task fits within the framework of the job title then you are not being unreasonable.
TUPE law can be a stupid law for reasons previously stated.
But then it is up to you to bring the cleaner into line and raise their standards, through consultation/training etc. if they will not raise their standards then you take them down the disciplinary route and eventually dispose of them if need be, as you would do with any other employee.