Employment Law Questions about Flexible Working/Homeworking/Work-Life Balance?

I requested working from home with my employer. They granted me 2 days a week on a 3 month Trail Period with the possibility to increase if everything was fine, or revoke if it didnt work out. Several others are also on these terms as well. The 3 months has elapsed in April and I am continuing to work 2 days from home as is everyone else whos Trial Period have also elapsed. There has been NO review of the trial for any of us... but again there has been NO indication that they are unhappy with the result as we are still doing this. Can I request this to now not be a verbal agreement and a change to my contract.. how long does a working practice have to remain in place before I can get it in my contract. If My emplyer refuses to update my contract and wants to keep it as a verbal agreement.. where do I stand? Can they make changes to it at anytime? Revoke it? Or do I have a right to say that it is my working practice and they cannot take it away? I do not have children, I do not care for anyone.... I was granted this on the basis that I my commute is 2.5 hours each way and an 8 hour day in the office meant my work-life balance was seriously affecting my wellbeing and health and also I could not work late or overtime because of the fact I had too far to travel etc.. I am concerned though without it being concrete in my contract they could take this away from me when they feel like and I will have my health affected again!