No. If you ask for a level of fitness in excess of the requirements of the job, you are imposing 'an apparently neutral position, criterion or practice' which 'puts or would put persons of a certain age group at a particular disadvantage compared with other persons', and which cannot be objectively justified: in other words, you are indirectly discriminating against people over 40 (and probably against women and the disabled, as well). If someone over the age of 40 were to apply for a job with you and get turned down, they could certainly argue that your fitness requirements were discriminatory, and an Employment Tribunal would find in their favour unless you could prove that they were not.
