You need to check your mortgage documentation to find out if there are any penalties for paying part of it off early. If there are you might consider waiting until they no longer apply or take the money from your Mother and put it in the best savings account you can find until you can repay without penalty.
If you've got no early redemption penalties then there is no reason why your Mother can't give you as much cash as she likes for whatever reason. If you've got siblings then it would be wise to get your Mother to let them know what she is doing and she needs to be sure she doesn't need the money to live off but I'm assuming she's already considered that aspect. The first £3,000 a year will have no inheritance tax implications and if she lives for 7 years then the rest of it will be free of inheritance tax as well. If she dies before the 7 years are up then part of the gift would fall into her estate and there may be inheritance tax to pay on it (depends on the value of the rest of the estate).