I have been offered a promotion within my company but the offer letter includes some changes to my existing employment contract. The main one which concerns me is a Mobility clause that allows them to change my place of work in the future.
Am I within my rights to ask them to remove that clause before I accept the promotion?
You can always ask. They can always say "no". The one thing you can not do is demand unless you're willing to forego your promotion.
If they say "no", see if you can narrow down the geographical area they have in mind. Will they want you to move to Shenzen, China? South Korea? Florida?
If you do sign that clause and they relocate, you may forego your severance package (or your unemployment benefits) if you refuse to relocate. At the same time, if you do sign that clause now and eventually relocate, they may choose not to pay you as much of the usual relocation package since you've already agreed to relocate.
So if you do accept some kind of relocation in your new contract, you should ask for the terms of the relocation to be spelled out (if they're not already spelled out in your employee handbook). Will they pay you for your moving expenses? Will they give you a salary bump for the inconvenience? Will they increase/decrease your salary depending on the standard of living of the new location? Will they pay for your lease if you need to break it? Or your children's school if you need to take them out early? How much notice will they give you? Etc.
Or if they don't want to answer all these questions, or tell you, that you won't have to relocate anywhere, you could take them at their word and strike out that clause yourself and then ask them to initial the change you've made.
Am I within my rights to ask them to remove that clause before I accept the promotion?
You are always within your rights to ask. They are within their rights to decline your request.
So before you ask, decide how you will react if they say "No".
