Employees with two employments

If an employee has two employments for example each paying £45,000 per annum, so neither on it's own would be taxed at higher rates but total earnings would be £90,000 per annum which would create liability to income tax at higher rates, how and when is the higher rate tax assessed and collected ?

Hi bernard lindberg,
There would be an adjustement in the main tax code to collect the tax due at the higher rate based on the estimated income. 
Thank you.