Without sitting down with an Adviser (so they can get your complete picture) its impossible to say what would be best for you. However a few thoughts:
First of all that's a lovely thing for your mum to do - so many people leave it to children for when they are gone, however if they can afford to do it when they are alive, it's much nicer for them to see it enjoyed.
There is really only one thing you can do apart from pay it off and that's invest it - now that could be a bank or building society account, the stock market or even property, endless options really. However they are all a risk and that risk is 'will they do better than simply paying it off?' I would imagine your mum wants to see your mortgage paid off and if that's the case maybe that's the best course of action.
One option that might be sensible is to do something with the monthly mortgage amount that you are saving. Reason being is if you don't 95% of people simply increase their outgoings and 10 or 20 years down the road are no better off whatsoever. Again that could be putting it towards property, stocks and shares, topping up your pension, national savings etc etc. I wouldn't use all of it as lets face it, life is for living today (as long as we plan a bit for tomorrow), so if say your mortgage is £500 per month, how about putting £250 away for the future and using £250 for holidays, nights out, watches - whatever you like! That's probably what I would do in your circumstances but as I said without the complete picture, very hard to say.