This what the letter of employment should include according to FM 1.7:
b) A letter from the employer(s) who issued the payslips at paragraph 2(a) confirming:
(i) the person's employment and gross annual salary;
(ii) the length of their employment;
(iii) the period over which they have been or were paid the level of salary relied upon in the application; and
(iv) the type of employment (permanent, fixed-term contract or agency).