Good luck! Now is the time to do it, and get it put into your contract. You have additional leverage as employers are panicked by stories of workers resigning en masse if not granted additional working from home time.

Don't fall for the "you can do more work because you're not commuting" line - if commuting is considered work time, then you deserve to be remunerated for that time, and they owe you back pay for each day you commuted in the past.